Helen Mirren: Her crowning achievement

It's the nose that sparks recognition when Helen Mirren enters. A slight figure with ash-blond shoulder-length hair in a pink cardigan, she might have been almost anybody except for the distinctive pointy nose that, with her head held high, seems to lead her on to whatever challenges lie ahead.

I have noticed how most interviewers pay homage to Mirren's glamour in their opening paragraphs, as if somehow duty bound to genuflect to the legend of her eternal sexiness. Without a hint of plastic surgery, and since last year old enough to collect her state pension, she's certainly wearing well. And there is a playful warmth and lambent responsiveness that could, I suppose, be called sexy. But at the moment I'm just grateful that Dame Helen Mirren isn't playing the dame.

"I can't bring myself to use the title or even believe it of myself," she says, not for the first time since Prince Charles ennobled her in 2003. Ninety minutes in Mirren's company and you fully believe her; "down to earth" is how I found myself describing her to anyone who asked. Directors must love her; indeed one, the American Taylor Hackford, married her. But more of Hackford later; for now, we must focus on the dame.

Mirren, renowned for a lack of squeamishness about nudity and a "bit of an old leftie", seems an unlikely candidate for - or indeed willing recipient of - Establishment honours. "I took it because of my parents, really - which was silly, since my parents were both dead by then," she says. "They didn't give a damn... They were anti-monarchists anyway."

In which case, Basil and Kit Mirren will now be spinning in their graves, as their daughter plays, back to back (in what must be some sort of a record), both Queen Elizabeth I and Elizabeth II. In the Channel 4 drama Elizabeth I, Mirren portrayed an earthy, sensual "virgin queen" of advancing years. Now, in the film The Queen, Mirren gives the most comprehensive and convincing portrayal yet of our present monarch, although don't expect any frolicking. The nearest Mirren's Majesty gets to a "bedroom scene" is a peck on the cheek from Prince Philip before lights out.

The Queen reunites the team behind The Deal, the acclaimed television drama about Tony Blair, Gordon Brown and the birth of New Labour: the writer Peter Morgan, the director Stephen Frears and Michael Sheen as Tony Blair. Their new film is being released in cinemas, premiering at the Venice Film Festival in early September and opening in the UK later in the month. It tells the story of the momentous week in the late summer of 1997 when Diana, Princess of Wales died in a car crash in Paris and the Royal Family nearly sleepwalked into disaster with its refusal to budge from Balmoral and to join in the hysterical mass mourning of "their" people.

Sheen was a certainty to play Blair again, the Cheshire Cat grin just right as he persuades the royals to unbend, show some public grief and salvage their position. It was Andy Harries - controller of drama, comedy and film at Granada - who spotted Mirren's royal potential. "I was at a read-through of the sixth series of Prime Suspect," Harris recalls, "and I was looking at Helen and thought, 'She's the queen of British drama and she looks a bit like the Queen..." Perhaps he had seen what a critic from The New Yorker wrote of Mirren: "Probably no other actress can let you know as fast and economically as she can that she's playing a distinguished and important woman."

Mirren herself, although terrified of the role - "I was more nervous about this role than almost any other" - took little persuading. "I thought The Deal was a fantastic piece of work, so I knew I would be in very good hands," she says. "It's delicate material, dangerous material in a way, so you have to be confident that the people you are working with have the intelligence and ability to put a story like this on the screen without a cheap betrayal of the subject."

The Queen presents by far the most detailed and complex screen portrayal of our present monarch, next to which Prunella Scales in Alan Bennett's A Question of Attribution looks like the deft cameo that it is. And, while playing Elizabeth I was largely a mixture of historical guesswork and intuition, Mirren says our modern queen was no open book either. "There is so much more intimate detail about her, but she's so private and always has been. I did a lot of research because you want the audience to believe in who you are and go on a journey with you in an imaginative way. I didn't want mere impersonation. I'm not very good at mimicry anyway and even if you are the most brilliant mimic in the world you'll only capture 50 per cent of what the real person is."

Mirren asked the advice of Sheen, whose Blair goes well beyond caricature into credible characterisation. Sheen advised her to work closely with the dialect coach Penny Dyer from as early a date as possible so she would feel comfortable with the voice and the mannerisms when shooting began. In the same spirit, Mirren decided to gather together the actors who were playing her family - James Cromwell (the farmer in Babe, and inspired casting as Prince Philip); Alex Jennings, who would play Prince Charles; and Sylvia Syms, the Queen Mother - at her house in Wapping "so that we got used to the sound of each other's voices as a family and it wouldn't feel like being with a whole group of strangers talking in funny voices."

The writer, Peter Morgan, was impressed by Mirren's transformation. "For the first few days, she was Helen Mirren in a wig," he says. "But then she started inhabiting the role more and more and suddenly she became this rather squat, piggy woman with enormous presence. She would walk on the set and you would find yourself minding your Ps and Qs."

Such was Frears' confidence in the finished article that he opens the film with Mirren in a portrait pose gazing off into the middle distance. Mirren's Queen slowly turns to the camera and stares at the audience. The effect is both playful and shocking, and a statement of intent. This is one of the most accomplished actresses on the planet, it says, and she is going to bring you the inner Elizabeth II.

"I had photographs of the Queen in my trailer and watched tapes all the time," Mirren says. "It was a bit intimidating because each time I watched them I would feel I was failing her, failing the inner person. There's one film - about a minute of Elizabeth as a girl of about 12 - getting out of a car and walking forward to shake someone's hand. I found it very touching. I watched it over and over.

"Our piece is enormously sensitive to her; it's not some nasty little bitch piece, it's really trying to get inside what happened within the Royal Family in the week after Diana died. The older generation - what I call my parents' generation - having to confront the Hello! magazine generation, with its fake grief."

For the record, Mirren's wedding in 1997 (after 13 years together) to Hackford, the Hollywood director of Ray and, most famously, An Officer and a Gentleman, was pictured in Hello! - albeit on very Mirrenesque terms. "When I got married to Taylor, we didn't have any official photographers - people brought their own cameras," she says. "I said to my nephew and his wife, who were a struggling young married couple, 'Take those photos if you want to and split the money.' And they did. So, yes, my marriage was in Hello!, but all terribly blurry pics literally taken with throwaway cameras." * * It's a story that encapsulates something about Mirren. Coming to maturity in the 1960s, there is something of her generation's disregard for conventions about this dame with a tattoo on her hand. "It was done long before it was fashionable," she says, showing me the arrows pointed in opposite directions, which she had done when she visited a Navajo reservation with Peter Brook in the 1970s. "I'm so horrified that tattoos should become bourgeois, middle class. But then I am bourgeois and middle class."

Well, yes and no. Mirren may be British, but she's British with a twist - she was born Ilyena Lydia Mironoff. Her paternal grandfather was a White Russian stranded in Britain during the October Revolution, while her father, who was two when he left Russia, became a communist taxi driver who lived in Essex. Mirren's mother was from Pimlico in London, when Pimlico was solidly cockney working class. "We were just poor middle class," she says. "Working-class money but middle-class attitudes."

Certainly, their aspirations for Mirren, born in the first post-war summer of 1945, were middle class. "They would have liked me to become a teacher, or a solicitor," she says. Instead, she joined the National Youth Theatre before graduating to the RSC in 1965 to become famously dubbed "the sex queen of Stratford". The rest, as they say, is history, from numerous leading stage roles to the nervy, chain-smoking Jane Tennison in Prime Suspect, by way of the classy tarts of The Long Good Friday and The Cook, the Thief, His Wife and Her Lover.

Did her parents live to see her success? "My dad saw me become a successful theatre actress, and my mum saw me become Jane Tennison," she says. "They saw me earning a living, being able to buy a house - the things that mattered to them most. They belonged to that noble generation - quite an incredible generation - that we're losing now."

Mirren's admiration for the stoicism of her parents' generation is perhaps strange, given that that was the quality that made it difficult for her to get beneath the skin of the woman she portrays in The Queen. "She is so incredibly iconic and well known, yet we don't know her at all. She's back within herself - not like Tony Blair, who's all forward."

And Mirren has observed Blair up close at a critical juncture in his career. It was her role in Prime Suspect that led to her being asked to campaign for Blair in 1997. "They wanted me to pose with a real policeman or something," Mirren says. "I've never been a member of Labour, or any political party for that matter, but in 1997 I wanted to get rid of the Conservatives; wanted to get rid of that appalling lot."

Mirren found herself on the Labour campaign bus on election day. "I sat in the front next to the driver, which I thought was a humble place to sit. But then Blair came up front so people could see him and I found I was sitting between Blair and the driver. He refused to let anyone say, 'We're going to win.' He refused to take anything for granted.

"At the same time you could see on his face that he could see this thing coming at him like an express train. And then he said this very curious thing; he said, 'I can't see myself doing this for ever' - and that was the day before he got elected. It's like he saw it coming, but wanted there to be a finite end to it."

Is she disillusioned with Blair? "No, I'm not disillusioned. He's a politician, for God's sake. If you're not allowed to change your mind, which you're not as a politician, which seems to me absurd, then they have to lie. That's why I never belong to any political party."

The old leftie has become something of an ageing cynic, or perhaps a realist; something that extends to events in Iraq. "You have to put your money where your mouth is - if you like all this," she says, waving a hand towards the window and its view of the London skyline. "It all depends on oil. Either we decide to wean ourselves off oil, and we don't invade Iraq because we don't need to, or we stop being such hypocrites. People love what oil brings. The same people who are against the Iraq war are the same who insist on driving their children to school, because they can't contemplate that their child should walk."

Happily childless herself, she is stepmother to Taylor Hackford's grown-up sons, Rio and Alexander. "Step-motherly? I was never very motherly... nor very 'step'," she jokes. Although Mirren and Hackford live in Tinseltown, it's not among the glitzy mansions of Mulholland Drive or Rodeo Drive, but on the fringes of funkier West Hollywood.

They also have homes in the south of France and in Wapping in east London, where Mirren stays when making Prime Suspect - a seventh film ("absolutely definitely the last") airs on ITV in the autumn. And, although she says London is her "spiritual home", Mirren feels her career is slanting towards America. She certainly thinks that some of her best recent work has been for American television - in Door to Door, with William H Macy, and Tennessee Williams' The Roman Spring of Miss Stone. She was Emmy-nominated for both.

"The recent stuff I'm proudest of hasn't been shown in Britain, which is sad," she says. Is she recognised on the street in America? "I would say I'm recognised more in America than in Britain. More people stop me, anyway. Over here, I get recognised for a few weeks after Prime Suspect has gone out, but that's it."

Mirren is 61 now, and a star, but she can still show a vulnerable streak. "I do say to my husband, 'I accept nothing but unconditional praise - especially after a show,'" she says. "He can tell me the truth five years later." But any insecurity does not seem related to her age. "That comes from my mother. She always said, 'Don't worry about getting older, darling, nature has a wonderful way of maturing your mental faculties so that you don't mind the physical side of ageing.' The physical disability of getting older must be difficult, but on the whole I just think that everybody else is getting older, so you're not the only one. You're with your group - you're sharing the same experiences."

But isn't the experience of getting older harder for a "sex symbol"? "I don't think this sex symbol thing, however flattering, is true. And anyway, it's dangerous, because if you start believing it, then they go, 'Oh, she used to be a sex symbol, she isn't any more.' I'm just who I am, I get on with my work, I do my thing and I do it as best I can. My job is my job."

The Queen herself couldn't have put it better.

'The Queen' opens on 15 September; 'Prime Suspect 7' will be broadcast on ITV1 in October