Actress Meryl Streep unveils a poster for her new film The Iron Lady

Actress Meryl Streep unveiled a poster in London on Monday promoting her upcoming big-screen portrayal of former British Prime Minister Margaret Thatcher, which one early review described as magnificent.

When details of the screenplay became public over a year ago, Thatcher's children were quoted in the press as being appalled at the concept.

Questions were also raised as to whether an American actress should play one of the towering figures of British politics.

The Iron Lady, which hits British cinemas on January 6, depicts an aging Thatcher looking back on the highs and lows of her career and the personal price she paid for power.

Streep attended the photocall against the backdrop of the Houses of Parliament, and was joined by the movie's director Phyllida Lloyd, who also worked with Streep on the hit musical Mamma Mia!

The Iron Lady is likely to be a major media talking point over the coming weeks in Britain, where former conservative leader Thatcher is still a divisive figure both revered and reviled by the public.

Streep, a two-time Oscar winner and one of Hollywood's most respected actresses, told the Daily Mail newspaper that playing Thatcher was the biggest role of her career.

It took a lot out of me, but it was a privilege to play her, it really was, the 62-year-old was quoted as saying.

It was one of those rare, rare films where I was grateful to be an actor and grateful for the privilege of being able to look at a life deeply with empathy.

Streep added that while she did not agree with many of Thatcher's policies, I feel she believed in them and that they came from an honest conviction.

The right-leaning newspaper's showbusiness correspondent Baz Bamigboye called Streep's portrayal magnificent.

It's a performance of towering proportions that sets a new benchmark for acting, a searing interpretation that looks at the big forces that shaped Mrs T's life, he wrote, using a common shortened form for Thatcher's name.