The family of Margaret Thatcher have rejected an invitation to see the Iron Lady at a public screening, the film's director said today.

Phyllida Lloyd said that despite basing much of the film on the autobiography of the former prime minister's daughter Carol, she had not spoken to any members of the Thatcher family about the controversial picture.

But she said she could understand why the Thatchers might want to "step back" from the project.

The Iron Lady has attracted strong criticism from some quarters for the way it has depicted Baroness Thatcher suffering from dementia, but Ms Lloyd said the performance of Meryl Streep "took care" of the former premier's "dignity".

Speaking on the BBC's World At One programme ahead of tonight's premiere in London, she said that half the film was "pure imagination", inspired by Carol Thatcher's book A Swim-On Part in the Goldfish Bowl.

The other half was told from Lady Thatcher's point of view and was a story about "power, and what it might have felt like to have great power and then to lose all power", she added.

Lloyd said: "I think most people who see the film will feel that Meryl's performance of the older Margaret really does take care of her dignity and we all felt that somehow the portrait of somebody who is experiencing a failure of strength and health and forgetfulness is not a shameful thing to put on the screen.

"We did make contact with the family sometime ago to tell them what we were trying to do but they perhaps quite understandably have sort of stepped back from the whole thing.

"They were the first people we invited to see the finished film. They didn't take up our offer and I can quite understand them not wanting to see it in the public gaze so we are not actually sure whether they have seen it or not."