Lucian Freud: Artist widely regarded as the finest painter of the human form in the second half of the 20th century

A visit to Lucian Freud's house in Notting Hill was an unforgettable experience.

Depending on the time of day, one might be offered green tea ("They say it's good for you"), lunch at Clarke's in Kensington Church Street, or vintage champagne, which, as the artist was invariably between models and had several hours of night-time painting ahead of him, he would not touch. Talk ranged widely, from life in Paris after the war to the latest exhibitions and art world personalities. Extremely articulate, with only a hint of a guttural "r" betraying his German origins, well-read, with a phenomenal memory for facts and anecdotes, as well as poetry, limericks and songs, Freud was one of the wittiest conversationalists of his generation. Finally, one would be invited to view work in progress, climbing to the first-floor studio, heated to an uncomfortable degree if you were fully clothed, its paint-encrusted walls and growing mountain of smeared white rags immortalised in photographs by Freud's assistant of 20 years, David Dawson.

Although he was widely, and rightly, regarded as the greatest painter of the human form in the late 20th and early 21st century, fame and success came to Freud relatively late. It was not until the late 1980s that he began to achieve anything like an international reputation, with shows in Washington, Paris and Berlin. This belated recognition may have been due in part to the revival of interest in figurative painting. Freud's adherence to a naturalistic tradition based upon intense observation of the visible world was seen by many as quaint and even anachronistic. But gradually his art, with its powers of concentration and empathy, and above all its poignantly human subject matter, won him more and more admirers.

By the mid-1990s Freud's work was rapidly becoming out of reach of museum budgets, affordable only to the very rich private collector. In 1998 his Large Interior W11 (after Watteau), 1981-83, a masterful group portrait of members of his extended family, was sold for over £3.5m at auction in New York. (It had been turned down 15 years earlier by the Tate Gallery, priced at £200,000.) In 2005 Freud beat his own record when his portrait of a Red-Haired Man on a Chair (1962-63) fetched £4.15m at Christie's in London. At Christie's again, in 2007, he became Europe's most expensive living artist when his full-length portrait of his friend Bruce Bernard sold for £7.86m. Less than six months later in New York, a large double portrait of his daughter Ib (Isabel) Boyt and her husband, fetched over £9m. And in 2008 Freud's Benefits Supervisor Sleeping (1995), a portrait of his model "Big Sue" Tilley, set another record for a living artist when it was sold at Christie's, New York, for over £17m, reportedly to Roman Abramovich.

Freud's realism was of a very different kind from Francis Bacon's, an artist from whom he nevertheless learnt a great deal. The two men were close friends, and Freud admired the older artist's panache and risk-taking in his life as well as in his art. Unlike Bacon, however, Freud never worked from photographs but only from the living model, scrutinising every nuance and inflection of his sitter's appearance close up. His penetrating gaze seemed to probe beneath the skin, exposing a landscape of muscle, bone, blood and flesh – each blotch, bruise and swelling recreated in a palette of grey-blue, yellow and red.

Freud's paintings make us uncomfortably aware of what it feels like to be human. His portraits of naked men and women, stripped of all protective trappings, reveal his subjects' animal sides but also show them as unique, vulnerable individuals.

Freud did not use professional models and rarely accepted commissions. His canvases vary in size from tiny heads measuring six by four inches to full-length and group portraits some eight feet high by four or five feet wide. These changes of scale, from smaller than life-size to larger than life-size, ensured that Freud never slipped into a formula: determination to avoid a method or "signature" was the guiding principle of his career.

Like great realist painters before him, Freud's subjects came from all walks of life: from working-class girls and art students to supermodels and the Queen; criminals and bookies to bankers and barons. He painted his wives, lovers, children and grandchildren; fellow painters John Minton, Francis Bacon, Michael Andrews, Frank Auerbach and David Hockney; the rich and powerful Baron ("Heini") Thyssen-Bornemisza and Lord (Jacob) Rothschild; members of the aristocracy such as Devonshires and Lambtons; and close friends including the photographer and picture editor Bruce Bernard, the art historian and critic John Richardson and the writer Francis Wyndham. Over the years he painted several searching and candid self-portraits. His series of portraits of his mother, begun after the death of his father in 1970, rank alongside Rembrandt's in their touching depiction of the isolation of old age.

Freud also developed a genre of portraiture exploring the vitality, even personality, of plants and animals, especially horses (he loved to ride as well as bet on them) and his own pet whippet Pluto. In spite of his reputation for naked portraiture he came relatively late to the nude. His depiction of unprepossessing, inanimate objects – whether the rubbish tip outside his studio window in Paddington or the floorboards, rough plaster walls, battered furniture and discarded paint rags in the studio – is unequalled in its loving attention to detail. From the late 1980s he worked on an increasingly large scale, reflecting his fascination with large people such as the performance artist Leigh Bowery and the latter's friend, the benefits supervisor Sue Tilley. At the same time he produced an impressive body of etchings – again, from life – which have been shown integrated with his paintings.

Lucian Michael Freud was born on 8 December 1922, the second of three sons of the architect Ernst Freud and his wife Lucie Brasch, in a large apartment in the exclusive Tiergarten district of Berlin, an area of embassies and spacious parkland not far from the Reichstag. His older brother Stephen Gabriel became a dealer in antique door furniture while his younger brother, Clement Raphael, was the writer and broadcaster Sir Clement Freud. (Each child was given an archangel's as his second name.) Their parents had met at Munich University, where their father was finishing his architectural studies, having served as an artillery officer in the Austro-Hungarian army in the First World War. Lucian's father Ernst was the youngest son of Sigmund Freud.

Lucian's mother was the daughter of a grain merchant. Summers were spent on the island of Hiddensee, a favourite haunt of artists and intellectuals off Rügen Island in the Baltic, or at Freud's mother's family estate near Cottbus (the subject of his daughter Esther Freud's novel Gaglow). The children were brought up by nannies and initially educated by governesses.

Their schooling in Berlin was cut short by Hitler's accession to power. In the spring of 1933, shortly after the Reichstag fire (which Lucian remembered watching with excitement), and only weeks before Sigmund Freud's works were burned in the infamous book-burnings, the family emigrated to England. More than 50 years would elapse before Lucian agreed to exhibit his work in Berlin, in a retrospective at the Neue Nationalgalerie. (It was from this exhibition that his minutely observed, tiny head of Francis Bacon, belonging to the Tate Gallery, was stolen.) But Freud's contempt for Austria, and the way the Nazis there had treated his grandparents and their relatives, meant that all attempts to persuade him to show in Vienna were rebuffed.

The Freuds lived first in a rented flat off Piccadilly, eventually settling in St John's Wood. In 1934 the brothers were sent to Dartington Hall in Devon, the progressive co-educational boarding school whose headmaster was influenced by Freudian psychology. As Clement recalled, "my parents did not pay school fees because we were Sigmund Freud's grandsons". Lessons being optional, and speaking virtually no English, Lucian spent much of his time on the school farm, where he learned to ride, developing a love and knowledge of horses that is reflected throughout his work; for a time he thought of becoming a jockey. Unable to read English, Freud was nevertheless fascinated by the shape and character of letters. His own handwriting, with its non-cursive letters, was highly distinctive.

Rebellious and with a reputation for getting into fights, Freud was asked to leave Dartington. In 1936 he spent a year at Dane Court, the prep school for Bryanston in Dorset, where he made linocuts of horses. At Bryanston, which he attended from 1937-38 before being expelled, he joined the Oil Painting Club and executed a handful of stone carvings, one of which, of a horse, has survived. It was this which helped him gain admission to the Central School of Arts and Crafts in London in 1938.

The following year he became a British subject and in summer 1939, gently encouraged by his adoring mother, enrolled in the East Anglian School of Painting and Drawing, run by the painter and horticulturalist Cedric Morris and his lover Arthur Lett-Haines at Dedham in Essex. After the school burned down – caused, it is alleged, by Freud and a fellow student's negligent smoking – it moved in mid-1940 to Hadleigh in Suffolk. Morris's direct and candid approach, as well as his unconventional method of painting, without drawing or revisions, made a deep impression on Freud, who attended the school intermittently until late 1942. Freud's portraits of this period, which include one of Morris, have a similarly naive, almost caricatural quality. His sitters were either fellow pupils or local boys from the village.

In 1941, Freud joined up as an ordinary merchant seaman on the SS Baltrover. After a few months sailing back and forth to Nova Scotia he was invalided out with tonsillitis. From 1942-44 he and the painter John Craxton had studios in a house in St John's Wood, paid for by Peter Watson, art editor and financier of Horizon and a collector of modern art. Watson was one of Freud's earliest patrons, commissioning work and publishing some of his drawings in Horizon.

Another friend and supporter was the poet Stephen Spender, associate editor of Horizon. Freud appears to have been befriended by a number of homosexuals prominent in the artistic, musical or literary world: Watson and Spender, and Christian Bérard, Boris Kochno, Olivier Larronde and James Lord in Paris and, back in London, James Pope-Hennessy (whose book The Baths of Absalom he illustrated), Richard Buckle (who published some of Freud's early drawings in Ballet magazine) and Cecil Beaton (who photographed him). Freud made portraits of all of them. Photographs of Freud at this period suggest strikingly romantic looks: a lean, intelligent, alert face crowned with a mop of dark curls. Ever the dandy, when not painting Freud favoured tailor-made suits in the finest cloth or cashmere, silk shirts worn with scarves of the most discreet hue, and hand-made shoes.

In early 1944 Freud moved to Delamere Terrace in a run-down part of Paddington, where he lived and worked for the next 30 years. It was at there that he painted The Painter's Room (1944), the most surrealist of his early works, which shows an enormous yellow and red-striped zebra's head intruding through a window.

After an intense affair with Lorna Wishart, 12 years his senior – the subject of incisive portraits in both oil and pencil – Freud spent the summer of 1946 in Paris. In 1948 he married Wishart's niece, Kathleen ("Kitty") Garman, daughter of the sculptor Jacob Epstein and Kathleen Garman. There were two daughters: Annie (born 1948), a poet, and Annabel (1952), a painter. (Annie's daughter May Cornet is also a painter.) The marriage was dissolved in 1953 but not before Freud had recorded Kitty's striking appearance – thick, dark hair, large eyes and frightened expression – in a series of outstanding portraits. With their sharp outlines and almost hallucinatory precision, they seemed to hark back to Northern Renaissance or early Netherlandish art. Freud's linear, graphic style was suited to etching, which he took up at this time.

In 1952 Freud eloped to Paris with Lady Caroline Blackwood, whom he married the following year. Freud depicted Blackwood – large, dreamy blue eyes, blonde hair and wide, enquiring face – in at least six oils, including two showing her in bed. They spent most of 1953 living in Paris in a cheap hotel on the Left Bank. The marriage was dissolved in 1957.

By this time Freud had tired of his quest for perfect line and surface finish and had begun to introduce greater modelling and depth into his transcriptions of the human figure. A change of brush from fine sable to coarser hogshair assisted in this process, resulting in directional brushstrokes of increasing spontaneity. By the early 1960s the paint itself had become richer, creamier, more sensual. In the 1980s the artist began to use a heavily leaded white pigment which appeared to coagulate on the canvas. It was around this time that he remarked to Lawrence Gowing, one of the most perceptive writers on his work: "As far as I am concerned, the paint is the person. I want it to work for me just as flesh does."

While his early work used to be seen in the narrow context of English Neo-Romanticism – a label he always rejected – as time went on Freud came to be regarded as the true heir to a European humanist tradition, engaging in a dialogue at different times with Old Masters such as Rembrandt, Hals, Chardin, Watteau, Ingres, Constable, Courbet, Van Gogh and Cezanne. Nowhere was this made more explicit than in the Freud exhibition at the Centre Pompidou in Paris last year: spacious, well selected, with works hung in themes, its final room of large, late nudes reminiscent of one of the grand galleries in the Prado or Kunsthistorisches Museum.

The 1950s saw Freud's work come into fashion. His large Interior in Paddington was exhibited at the Festival of Britain in 1951, where it won a prize. A full-length portrait of the photographer Harry Diamond looking tense and wary standing beside an enormous, threatening potted palm, it somehow caught the postwar mood. The coldness and hint of alienation associated with such paintings led Herbert Read to call Freud "the Ingres of Existentialism". In 1954 Freud represented Britain at the Venice Biennale, with Francis Bacon and Ben Nicholson, and in 1958 had his first show at Marlborough Fine Art.

The 1960s, by contrast, were financially leaner, with fewer critics and collectors prepared to back Freud's move from a linear to a painterly style. "Lucian Freud is one of the most serious painters at work in Britain today," wrote John Rothenstein in 1967, "but, in spite of his talents, one of the least regarded." His first retrospective took place at the Hayward Gallery in 1974; but it was his second, in the same venue 14 years later, that made the more lasting impact on a gallery-visiting public now largely re-educated to appreciate figurative painting.

For most of his life Freud saw off all approaches from aspiring biographers, photographers, film-makers and art historians. However, for someone who so fiercely guarded his privacy, he attended a surprising number of private views and parties and in his later years gave a number of revealing interviews. In 2010 Martin Gayford published his Man in a Blue Scarf: On Sitting for a Portrait by Lucian Freud. A first-hand account of how a portrait by Freud slowly evolves – 40 two-hour sittings over eight months for a head measuring 26 by 20 inches – it is also an invaluable record of the artist's unorthodox views on painting, as well as an affectionate portrait of the artist, with amusing descriptions of the dance-like movements, gestures, sighs, muttered phrases and surprising exclamations Freud made while painting, which he always did standing up. Sharply observed pen portraits of Freud are also to be found in his daughter Esther Freud's novels Gaglow and Love Falls.

In recent years Freud increasingly used his assistant David Dawson as a model. In turn, Dawson documented life in Freud's studio in a sequence of vivid photographs. The older Freud became, the more openly he talked about his youthful promiscuity, his attraction to Soho low life, his addictive gambling. His private life was the subject of speculation and gossip in the media and he resorted to law on more than one occasion. Charismatic, irresistible to women, he had numerous relationships. He is survived by the two daughters of his marriage to Kitty Garman, and by a number of children from various liaisons, including the fashion designer Bella Freud, her sister, the novelist Esther Freud, and the novelists Rose Boyt and her sister Susie Boyt. Until recently, he could be seen dining out most nights with one or other of his daughters at the Wolseley in Piccadilly.

Lucian Michael Freud, painter: born Berlin 8 December 1922; CH 1983; OM 1993; married 1948 Kathleen Garman (divorced 1952; died 2011; two daughters), 1953 Lady Caroline Blackwood (divorced 1957; died 1996); died London 21 July 2011.