It's not a detailed re-enactment of Nash's life

Ron Howard

But the director has said he only ever intended to use Nash's story as a "springboard" from which to construct a film, which was never supposed to be a true account of Nash's life.

"This is not a biopic. This is not a story where we're re-enacting all the details of a famous character's life," Howard told BBC's Breakfast programme.

"We're using John Nash's life as a sort of springboard to illustrate something that is quite difficult, but in this case we thought was quite interesting."

The main point of the film was to illustrate how someone could live with schizophrenia, Howard said, and Nash was used as an interesting example to tell a wider story.

Crowe could get an Oscar for his role

"It's not a clinical representation of the disease and it's not a detailed re-enactment of Nash's life," he said. "But we did use the headlines and the shape of the life to build the story on."

A Beautiful Mind is odds-on favourite to win the Oscar for best picture, while it has seven other nominations, including best director for Howard and best actor for its star Russell Crowe.

Nash was a maths prodigy who was institutionalised after developing theories that later won him the Nobel Prize for Economics in 1994.

But a film critic wrote in Friday's Daily Express that Howard's version "is taken up with characters that never existed and bizarre fantasy sequences that are in no way related to the sort of delusions Nash experienced".

Howard praised Crowe's creative integrity

Howard responded by saying: "It's not a documentary, I'm not a documentarian, I'm a story-teller.

"With stories inspired by a real event, you need that element of truth as a springboard.

"If a story is as unusual as this one, you also need to be able to say this really did happen, it's not some sort of flight of fancy, but what you're really doing is you're looking for a dramatic theme or an idea, in this case it's illustrating mental illness."

"Our big dramatic objective, quite frankly, was to try to allow the audience to understand what it might feel like to go through this experience - not to be clinical about it."

'Talent'

He also said he kept Crowe and Nash apart "for a while" because he did not want the actor to feel as though he had to impersonate Nash.

Howard heaped praise on Crowe's "power and range" of talent, saying he had a great deal of creative integrity and was not afraid to challenge him over things he thought were being done wrong.

Howard, still known to many as teenager Ritchie Cunningham in 1970s TV hit Happy Days, also directed Splash, Parenthood, Backdraft, Apollo 13 and The Grinch.

He will be at the Bafta Awards in London on Sunday, where he is nominated for best director and A Beautiful Mind is up for best film.