Jane Birkin - Sixties sex symbol, chanteuse, actress and, latterly, French icon - confesses early in my interview with her that she is jealous of her bulldog Dora. Though both are international travellers, Dora has one distinct advantage: rather than being plagued by a passport, she is blessed with a small injected microchip. "I would love to have a chip," Birkin declares, "I wish they'd wham one in straight away. I'm always losing passports, so I'm constantly in that awful queue when you think, 'Oh no, not at my age, not again.'"
It is difficult to believe that, at 58, Birkin thinks anything is inappropriate for her age. It was 1966 when she made cinematic history by appearing as a full-frontal nude in Michelangelo Antonioni's Blowup - "I only did it for a dare" - and 1969 when she shocked the world by recording the orgasmic "Je t'aime" with her lover Serge Gainsbourg. Almost four decades later, she retains her gamine figure, while her dark hair is swept up chaotically around an animated face, dominated by her famous gap-toothed smile. When we meet at the Sloane Hotel in Chelsea, she reveals that she has also kept her talent for upsetting world leaders. In 1969, it was the Pope; now it is the Russian president, Vladimir Putin, who recently banned her from performing in Russia "because I stuck up for the Chechnyans".
For someone who has whipped up so much controversy in her life, Birkin seems strikingly unworldly. Her sentences tumble out in long streams of consciousness, and she is happy to come across as terminally distracted. She tells a story about her portrayal of a countess - in a drama by Marivaux - who falls in love with a girl dressed up as a boy. "The director made me put my hands right against the actress's chest to show how in love I was with the character she was playing, so the actress said: 'Don't you think Jane will notice that I've got a pair of breasts?' And he said: 'To be honest, I don't think Jane will notice anything.'"
Yet Birkin notices a lot more than she pretends. Her hectic work schedule is testimony to the grit that is hidden beneath her eccentric exterior. In the four decades since her first cinematic appearance, she has notched up more than 72 films (some terrible, some impressive); her singing career continues to take her all over the world; and after being courted by more than one director, she is in Britain to play Gertrude in Hamlet. "I've been asked to play Gertrude three times in the past year," she says. "This time I knew that the director, Rupert Goold, had done a production of Paradise Lost, which I'm told was extraordinary - and fallen angels is one of my favourite themes."
Gainsbourg, who died in 1991, haunts the conversation. Birkin was briefly married to the prolific film composer John Barry in the Sixties, and left Gainsbourg for the film director Jacques Doillon in 1981, but it is the iconoclastic French musician who continues to shape her life. For her current tour as a singer, she is performing some of Gainsbourg's songs scored to Arab-influenced music, and even though none of them is causing as much outrage as "Je t'aime" or "Lemon Incest" (which he infamously performed with their daughter Charlotte), she has received much acclaim for the way she has sustained his legacy.
Birkin compares her former lover to the critic, experimental poet and pornographic novelist Guillaume Apollinaire, and, more surprisingly, to the Trojan hero Hector. "When I played Andromache [at the National Theatre in Women of Troy in 1995], there was my hero with all his armour and stuff behind me - it was just like my baggage with Serge."
Her appearance in Women of Troy led to an unexpected odyssey. Fascinated by the pronouncement of the director Annie Castledine that, in times of war, "when there's nothing left there are words", she volunteered to go to the Andre Malraux Cultural Centre in Sarajevo to take books to people living there under siege. "I also asked my mother, 'What did you take when your apartment was bombed in the Second World War?' - and she answered 'Schiaparelli Shocking perfume. When you're losing everything, it's what makes you feel good that counts.'
"So I went down the rue de Passy [in Paris] with my daughter Lou and chose a load of silk underclothes for the university students, and then I went to Guerlain and picked up all the lipstick, because I thought, 'They used to buy their shoes in Italy, probably Venice, so they won't want any cheapo things.' And Ma had been completely right - the shrieks of joy when we went to this ghost town and I pulled the frivolous things out of my backpack and flung them all on to the ground."
Birkin's mother, Judy Campbell, a former muse to Noel Coward, died last year. All the family were around her, including Birkin's three daughters, Kate Barry, Charlotte Gainsbourg and Lou Doillon, and their children. "She put on pearls and silk pyjamas because her grandson was coming and they opened champagne, and when the cork went off she said: 'I hope you haven't hit one of my nurses.'"
Birkin's mother was performing in the theatre only months before she died and, on current evidence, it seems that Birkin will continue to work for as long as she has the power to breathe. When she finishes Hamlet, she will be making a film about Robert Louis Stevenson - "I'm playing his wife" - and she has Electra lined up at the National Theatre in France. She finds it amusing and slightly exasperating that, despite all the work she still does, people seem most interested - after asking about Gainsbourg - in the famous bag that Hermes designed in her honour after she accidentally dropped her possessions over the designer when they sat next to each other on a plane.
"People always want to talk about it," she laughs. "I just don't understand. It can get a bit much" - and her eyes flash - "to be upstaged by a bag."
Hamlet starring Jane Birkin is at the Royal Theatre, Northampton (01604 624 811) from 17 March to 3 April