Ossie and Celia: the sequel

This painting will be one of the big draws at a new David Hockney retrospective in London. The story of its subjects, Celia Birtwell and Ossie Clarke, reads like an extraordinary Hollywood biopic

Celia Birtwell's face became synonymous with the Seventies; a portrait of her failing marriage hung on countless walls. Voted one of Britain's 10 greatest paintings, Mr and Mrs Clark and Percy will be the centrepiece of a major retrospective of the work of David Hockney at the Royal Portrait Gallery in London next week. And his subject will be there among the crowds. "It is extraordinary that it has become so popular," she says. "It is quite an odd image."

After decades of anonymity she has suddenly regained her status as a fashionable designer, as she was when the picture was painted in 1970. Then Birtwell was 29 years old and in a partnership whose clothes were defining the look of post-Swinging London. She printed textiles by hand and her husband, the designer Ossie Clark, turned them into fabulous and flamboyant outfits worn by stars such as Julie Christie, Marianne Faithfull, Elizabeth Taylor and Mick Jagger. When the couple split up in 1973, Birtwell turned her back on fashion and fame - but this year she made a remarkable comeback.

A range she designed for Topshop sold out within a day of its release in April and a second collection for the chain store is one of the most eagerly awaited of the autumn. Fashion is revisiting the early Seventies, and her clothes are being worn by Kate Moss, Sienna Miller and Helena Christensen.

"I didn't think that this generation had heard of me," she said. "I thought I'd been packed away for ever, but it gives me a real buzz when I see somebody wearing one of my prints on the street. People say that I'm finally getting my just desserts." Although the new exhibition covers more than four decades of the artist's work, the most popular work on show will be Hockney's tense portrait of the couple and one of their cats. Normally on show at Tate Britain, it is one of the gallery's most-viewed works and most-bought postcards. Last year it made the shortlist in a Radio 4 poll that was topped by Turner.

Birtwell and Clark met David Hockney in 1969, three years after they had launched their boutique Quorum in Chelsea, the heart of bohemian London. Professionally, the partnership was perfect. Their personal life was less happy. "It was a meeting of minds creatively, but personally, well ... quite a few people were damaged by the Sixties."

His drug use and affairs with both men and women led to the collapse of their marriage, and tipped him into a downward spiral of addiction and bankruptcy. Clark was living alone in a council flat when he was murdered by a lover in 1996. After separating from him Birtwell opened a shop selling furnishing fabrics, before collaborating with the French label Cacharel in 2002. Now in her mid-sixties, she admits to enjoying her second wave of adulation. "It has taken me by storm. If Ossie were around he'd be proud to see we'd resurrected his wonderful shapes. I just hope the next collection is as well received."

The painting was one of a series of double portraits Hockney completed in the early 1970s, but none of the other canvases has been subject to such intense analysis. Against the backdrop of a half-shuttered French window, the portrait shows a petulant Ossie Clark lounging in a metal-framed chair with a white cat on his knees and his feet buried deep in the cream shag-pile carpet. To the left of the window stands Birtwell in a flowing pink-and-black gown. She has her hands on her hips, and a long-suffering expression on her face. It is an image which seems to lay bare the tensions in the couple's relationship - both with each other and with the artist: Hockney had been a witness at their wedding, but he was also a lover of Clark.

Art critics claim to have deciphered hidden meanings in almost every aspect of the painting's content and composition: the cat is seen as a symbol of infidelity, while the vase of lilies has been interpreted as a symbol of Birtwell's impending pregnancy. (She and Clark had two sons during their five-year marriage.) Birtwell's recollections of its creation are more prosaic. "I remember that he had a lot of problems trying to get Ossie's feet right, so he stuck them in that rather horrid carpet."

Birtwell has often posed for Hockney, and she admits his most famous portrait is not her favourite. "It's a terrific painting, but it makes me look heavy," she said. "I prefer the crayon drawings David did of me in France where I look wonderful, young and gorgeous."