By James Mottram

It's comforting to know that even the most confident of us, even the most successful, have their moments of doubt. Take Helen Mirren, an actress you wouldn't normally associate with anything but supreme authority. "It's funny," she says, settling into her chair when we meet in a Berlin hotel. "When I was going through a period of great insecurity and fear about the future, I went to have my palm read because I wanted to be reassured. I was in my early 20s - 21 or 22. And I just wanted to be told that I'd be successful."

In truth, it's a delicious thought. Mirren is so imperious on screen, acting royalty who has made playing monarchs a particular specialty, it's amusing to think that she might turn to a fortune teller. Put it down to her "Gypsy sense of adventure", as she once put it. She's also had her time consulting the I Ching - the ancient form of Chinese divination. But in this case, she was merely given a piece of paper and pencil by her palmist. Told to write down everything he said, she scribbled down his stream-of-conscious predictions.

"I walked out," she says, pausing for dramatic effect, "and I couldn't remember what I'd written down. I had no idea. And there on the paper was my future. It was all in there somewhere. I looked at it and thought, 'You know what? I don't want to know.' And I found the nearest rubbish bin and I threw it away, because I realised that I wanted life to be an adventure. I didn't want to know what was going to happen. I wanted it all to come as a surprise. I thought, 'How boring to know what's going to happen.' That's horrible. I want to make my mistakes."

It would have made fascinating reading. Perhaps hinting that she'd collect a trophy-cabinet of awards: two wins in Cannes for Best Actress, two Emmys and three BAFTAs as the star of Prime Suspect, three Golden Globes and an Oscar for playing Elizabeth Windsor in Stephen Frears' The Queen. Maybe it'd say that she'd marry late in life, to a well-regarded Hollywood director - Taylor Hackford (The Devil's Advocate). Or that she'd never have children of her own (the world is already over-populated, she once said: "It is my contribution to ecology"). A star on the Hollywood Walk of Fame, a Dame-hood...I could go on.

Lauded, loved and lusted after, Mirren's career is enviable. But did she become the person she wanted to be? She goes silent for a few seconds, contemplating. "I think when you're younger, you don't really grasp what the possibilities are, if you know what I mean. You're so confused in your sense of identity and ego and insecurity and shyness - who am I going to be? It's very difficult when you're young to have a clear idea of the person you want to be. But if my young self were standing next to me, I would say 'Don't worry, it'll be fine.'"

Mirren doesn't come across as grand or theatrical. She's almost business-like, today wearing a long-sleeved black dress, gold earrings and a flash of red lipstick. Around her is a turquoise necklace - one she found in a jewellers in New York. "It's a bit Klimt-esque," she laughs - as if she's worn it deliberately. Her latest film is Woman In Gold, a true story about art restitution involving the paintings of Gustav Klimt, the Austrian-born artist who worked prominently in the late 1800s and early 1900s. "When I was a student, Klimt was the poster that you had on your bedsit wall," says Mirren, smiling at the memory. "We stuck him up with chewing gum on the wall - we were all big admirers."

Woman In Gold uses Klimt as a springboard into a moving tale that casts Mirren as Maria Altmann, a real-life Austrian who fled her country for California, leaving behind loved ones, as the Second World War began. For years, Altmann battled the Austrian government in court in an attempt to retrieve several Klimt paintings that belonged to her family. Stolen by the Nazis, the works remained in Austria after the war - including the famed Portrait of Adele Bloch-Bauer I, a painting of Altmann's aunt, a regular model for Klimt. "It's a story about justice, about perseverance," says Mirren, "and it's about something so very substantial."

A real-life David v. Goliath tale, the film shows how Maria meets a young lawyer with his own Austrian heritage - Randy Schoenberg (Ryan Reynolds) - to take up her case. "I think the story has a meaning for everyone, but for Jewish people watching this story, it's going to have a profound feeling," says Mirren. "My agent in Los Angeles, for example, found it incredibly powerful. Anyone who has this in their family background, obviously it's going to speak very powerfully to them. But to the rest of us, it's a powerful reminder. It's a story about humanity really, what people struggle with in life."

The film reunited Mirren with director Simon Curtis (My Week With Marilyn). He'd worked with Mirren years before, making her tea when he was an assistant director on a stage production of Measure For Measure in the later 1970s. This time, however, they went in as equals. "It's no surprise to me that she's as successful as she is," he tells me. "Before we started filming, I spent a couple of days going through the script with her. It was so interesting, seeing her instinct for the scenes and suggestions for new lines and so on. More - how to make the most of a moment."

A typically studied performance - speaking lines in German, her lips relishing Altmann's undiminished Viennese accent - Mirren calls it "a great female role for a woman of my age" before adding "and there are not that many of those around". While the film suffers from over-ambition, juggling Mirren's segment with flashbacks to Maria's younger days and even scenes of Klimt at work, the strongest element is the bond between Maria and Randy, who gambles his entirely livelihood on her case. "They loved each other - not sexually or anything like that. But they loved each other as human beings," nods Mirren.

Like Maria, Mirren understands what it means to return to your roots. She was born Ilynea Lydia Mironoff, and her grandfather was a Russian noble and diplomat who found himself stranded in England - where he was on a mission negotiating an arms deal - as the Russian revolution took hold. He reluctantly decamped to Leigh-on-Sea in Essex, where he lived until his death in 1955. Mirren, who was called by her birth name as a child, remembers being regaled by her grandfather's tales of their Russian heritage. When he died, her father anglicised their names - the Mironoffs became the Mirrens and she became Helen. "He thought 'Assimilation - forget all that. We're English now.'"

Yet Mirren never did. She always kept a stack of letters belonging to her grandfather, written by his six sisters, who has remained in Russia after the revolution. When she made the final Prime Suspect, her last outing as the weary workaholic detective Jane Tennison, there was a Russian actor in the cast. Out of curiosity, she brought the letters and an unfinished memoir written by her grandfather onto set and had them translated. Discovering the resting place for her great-grandmother, on a subsequent trip to Russia, Mirren visited the cemetery. When the story got out, she was contacted by a local journalist who uncovered both the location of her family's estate and some long-lost (and still living) relatives.

"Going to my family estate was pretty extraordinary. It was mythological in my mind. I never thought we would ever really find out where they were, although my grandfather actually left very good maps. I didn't know quite where it was." Sadly nothing is left - "no house or anything" - but just venturing on her family plot was thrilling. There, she found the tombstone of her great-grandfather. "Sometime in the revolution or in the war, it had been uprooted from its place in the cemetery, and it was just lying on the side of the road. And had been lying there for the last however many years. Apparently, people use it as a bench! And there is my great-grandfather's name on the tombstone! Pretty amazing."

When Mirren was a little girl, her life wasn't quite so grand as those led by her Russian ancestors. The second of three children, she describes her family as poor middle-class; her mother came from a family of butchers from the East End, which she says, rather played well when she wound up as Bob Hoskins' moll in the classic gangster film, The Long Good Friday. "I'd had an uncle - not a blood relative, but an uncle by marriage - who was an East End gangster. So I'd had tenuous connections with that world. For a long time afterwards, I had incredible credibility in the East End. Any East End pub I went into, any taxi driver...I had great cred."

Long before Mirren was born, her father had played viola in the London philharmonic before the Second World War broke out. Later, he drove a cab, before becoming a driving-text examiner and later still a civil servant in the Ministry of Transport. Basil - as he renamed himself - was an avowed socialist, who fought Mosley's Black Shirts in the East End. "My family were very anti-monarchist," she says. "They thought it was all a load of old rubbish. They hated the class system. So I was brought up not to believe in any kind of a class system."

While there is a certain irony that Mirren played Elizabeth I of England (in a Globe and Emmy-winning performance for a 2005 TV mini-series) and later the Queen, she defiantly nailed her colours to the liberal-left mast, campaigning against everything from the arms trade to Burma's military dictatorship. She even went on, unsuccessfully, to stand for the Workers' Revolutionary Party during elections to the council of Equity, the actor's union. Her work, too, has held a political slant - from playing the mother of a hunger striker in Some Mother's Son to the forthcoming Eye In The Sky, a thriller set in the world of drone warfare.

After leaving school, Mirren was admitted into a teaching college, studying speech and drama, but by the age of 18, she successfully auditioned for the National Youth Theatre. Two years later, she was playing Cleopatra in a production of Anthony and Cleopatra at the Old Vic before becoming a staple at the Royal Shakespeare Company. It was a heady time. "I was ambitious as a young actress. I wanted success without a doubt," she says. "I wanted my voice heard as an actor. I wanted my art to be recognised and all the rest of it. But I had no idea whether that was going to happen or not. You don't. Life is an unknown."

In her early days, Mirren dated photographer James Wedge, living with him for four years, often modelling for his experimental works. She's never been prudish when it comes to her body, frequently appearing naked in everything from Caligula to Peter Greenaway's The Cook, The Thief, His Wife and Her Lover. In 1995, next to the headline 50 and Fabulous" she posed (tastefully) on the cover of the Radio Times, à la Calendar Girls, a film she later appeared in as one of the clothes-shedding W.I. members. "I'm a naturist at heart," she claimed.

More recently, there was a snap of her holidaying in Puglia in a red bikini - looking stunning at 62. This year she turns 70 - her hair is a silvery bob now, the skin no longer as supple as it once was - but she's still in remarkable shape. "That is a sexy fox right there," grins her (much younger) co-star from Woman In Gold, Ryan Reynolds. "She isn't intimidating, she's accessible - and that's why moviegoers love her and that's why men fall in love with her when they see her on the street. She's this approachable beauty."

She's also still passionate about the profession, currently enjoying a stint on Broadway in The Audience, which she first performed in London in 2013. Directed by Stephen Daldry, it's a reunion with both the subject and writer - Peter Morgan - of The Queen. Again, she's playing the Queen. "It's a very different piece...it's more to do with the prime ministers actually," she says. "It's to do with the audience that the Queen has every week with the prime minister, so you see all the different prime ministers she goes through from Churchill to Cameron." The reviews have been predictably ecstatic.

Mirren is very careful with her words when it comes to talking about her portrayal of HRH. "Certainly the response I had - for example Prince William - and from members of the Royal household [has been] very positive response. I will never know what the Queen thinks. Even if I did know, I would never tell you! But I assure you, I don't know. The work that I did was heartfelt and I did what I thought was the truth. It's only a portrait - she's been very free with portrait painters. She's been painted looking twenty years older, she's been painted looking gloriously beautiful, she's been photographed by Annie Liebovitz. And I think of my work as simply another portrait."

As modest as this is, Mirren's "portrait" was a sensational depiction of the monarch at her most vulnerable, in the wake of Princess Diana's death. It led to Mirren's annus mirabilis - a clean-sweep of every major acting award going, culminating with a Best Actress Oscar; nominated twice before and once since, she compares her win to "a lovely dream". She keeps it in London, one of several homes she and Hackford share, though she's not one for hoarding mementos. "I'm not very good at keeping things," she says. "No, I'm not very sentimental actually. I want to get rid of it all. I find it depressing!"

This may well be the actress in her, always ready to shed a skin and move on. Fellow star Charlotte Rampling recently said that it must be scary to be Mirren because she always goes so far from herself. "It's not scary to be me!" she laughs. But is that true? Does she really disappear into her characters? "I don't know," she says, shifting a little uncomfortably. "Possibly. I don't know. It's certainly not conscious. Obviously when I play real-life characters, I have to."

From author Ayn Rand to Hitchcock's wife Alma Reville to Maria Altmann, there have been plenty of those. She's just played famed Hollywood gossip columnist Hedda Hopper, alongside Breaking Bad's Bryan Cranston, in Trumbo, a film about blacklisted screenwriter Dalton Trumbo that's already being talked up as a major Oscar contender for next year. Was it testing? "You're always testing yourself," she concludes. "The jobs that you sign up for are always the ones that scare the shit out of you." Just maybe it's written on her palm.

Woman In Gold (PG) opens on April 10