Let me paint you a story

Profile; Paula Rego; Marianne Macdonald applauds The Tate's celebration of a cruel but vibrant vision

Paula Rego tells a story she was raised on. A rabbit comes home from the shops to find a goat has taken over his house. The rabbit asks all the other animals to evict him, but the only one that will help is an ant - which climbs through the keyhole, burrows into the goat's belly button, and kills it.

Rego's paintings are full of such incidents of casual violence and revenge wreaked by animals/humans on one another - and full of fairy stories, too, for Rego has never lost her childhood amid the congealing certainties of adult life.

The greatness of her art, as visitors to her retrospective at the Tate will see from Saturday, is that it shows this unflinching child's gaze on the often dark complexities of human relationships: between mother and daughter, sister and brother, husband and wife. It is the more disturbing that the truths they reveal are often played out between humanoid dogs, bears and monkeys; or between humans and animals, locked in embrace.

One of her favourite pictures combines the childlike and cynical in unforgettable juxtaposition. A red woman-figure brandishes a huge pair of scissors, with which she has cut off her monkey-husband's tail. He is vomiting with horror, while the eye of a white bear looms in the corner, wide with fear.

Rego will tell you the key to her paintings are stories but equally important was her Portuguese childhood, where she grew up an only child, without television or friends of her own age, whom she shrank from in fear. "Children are a terrible threat," she says now, her face alight. "You think they are going to kill you, and they try to, don't they?"

As a young child, Rego was left for a year and a half when her parents went to England. Consigned to her grandparents' loving care every other week, the rest of her time was spent in the flat of a depressive aunt who sat all day long in an armchair.

John McEwen, who wrote Paula Rego's pictorial biography (published by Phaidon), ascribes the start of her artistic revolt to this experience, a revolt apparent later in life in her acclaimed dog-women paintings and fearful reworkings of fairy tales. He says that while Rego felt obliged to please adults as a child, she felt anything but demure inside. So she answered back through her art, and still does, giving form to her demons.

There were many of them, too. A generally cossetted child, even today she is agoraphobic, working with the windows of her Camden studio covered.

"My mother tells me I was afraid of the flies, but I remember being afraid of everything. I was even afraid of other children. I just couldn't bear to be put outside. Oh God, it was awful. It was just terror, terror," Rego recalls in McEwen's book.

This fear, and a vocation apparent in early childhood, pinned her to the drawing-board. Although Rego's mother had been to art school, her father was not artistic: Jose was an electronics engineer who owned a factory making precision instruments. Life with her parents in Lisbon was formal: it was important that she was turned out in white gloves, with well-brushed hair. But the young Paula was surrounded by verbal art: story-telling. Her grandparents, aunts and servants all told her stories and they have supplied much of her artistic fuel ever since.

Rego was taught from the age of 10 at an Anglican English school near Estoril and her talent for art was immediately spotted by her teacher Margaret Turnbull. "The boldness of the line, the strength of colour; to tell the truth it didn't look like the work of a girl at all, or a child come to that," she said later. It was Turnbull's encouragement that paved the way for Rego to attend the Slade in London.

At the age of 17, Rego started at the Slade, then at the height of its post-war popularity. It was 1952. Her husband-to-be, Vic Willing, 24, was a fellow-student: very glamorous, sexy and cool. Rego sinks into a slow reverie when she recalls him. "He always clicked his fingers when he walked - he walked clicking his fingers. He had straight shoulders, and he moved, when he wasn't ill, like a dancer. He was a marvellous jiver and he liked being apart from his partner when he danced, he liked to keep at a distance..."

But Willing, who went on to produce his best art in his last years, despite being immobilised by illness, was married. It was not until 1959 that they tied the knot after the birth of their first child, Caroline. By then they had moved to Portugal. They settled in her grandparents' villa in Ericeira - "I love that house! I've painted every room in that house!" says Rego - and worked in a barn divided in two by a curtain.

But while in some ways life was idyllic, the loneliness began to tell. In 1962 the couple returned to London with their three children and bought a house in Camden. Photographs from that time show Rego a poised beauty, hair framing her face in a bob and Vic, tanned, in V-necked jerseys. Rego was to get British recognition late, perhaps because she was willing to play second fiddle to her husband. Her first breakthrough came in 1965 when she became famous overnight in Portugal with her first solo show.

The vitality of the collages she exhibited caused uproar - "You must be a slut to paint these pictures," one visitor told Rego - but they put her name on the map. Happiness was soured by tragedy, however, when Vic was diagnosed with multiple sclerosis two years later. As time passed he would become more crippled and the strain of caring for him, then losing him in 1988, generated an increasingly personal note in her work with its images of suitcases and trunks. She lacks affection for the work from those dark years between 1966 and 1979, the year she was forced to sell the beloved villa which had belonged to her grandparents and where she spent the summers of her youth.

It was not until the early 1980s that Rego turned from collage and in doing so allowed her vision to fly free. This was the era of Wife Cuts Off Red Monkey's Tail and the other red monkey pictures; it was also the time of many of her other tour de force animal pictures such as Pregnant Rabbit Telling Her Parents - a painting of a pregnant girl-rabbit explaining her predicament to a stern dog-father and fearful cat-mother. Nor should one forget Rabbit and Weeping Cabbage in which Rego immortalised her mother as the cabbage. When she told her of this, her mother exclaimed: "Darling! You've made me look so young!".

Rego talks about these pictures with charm and honesty. One called Cabbage and Potato is also her mother, she says, although her mother never ironed, "and that stupid potato, he's cut off his nose with the knife". She points to another, Going Out, where a girl is applying lipstick and a dog is staring furiously in another direction. "He's cross because she's going out. They always want them to do what they want."

Rego's mobile face can switch from joy to thoughtfulness in seconds. And when she smiles, she becomes quite beautiful; she smiles a lot. So it is odd that her work can be so cruel - unforgiving in its honesty. L'Amour, one of her latest pictures, shows a woman, neck thrown right back, the beak of a huge bird thrust down it. She is drinking the bird in and holding him, while his claws are sunk deep into her side.

"How much is your work about revenge?" I ask, and her wonderful smile lights up her face again. "Always! Always! If you're a writer you can kill people off! I can cut people up - eviscerate them! And I can make them small!" And this, for those who have wondered at the incongruously tiny figures that scatter Rego's art, is the ultimate humiliation.