A British computer expert hacked into US military networks through "naivety" as a result of his Asperger's Syndrome and should not be considered a criminal, an expert said today.

Gary McKinnon, 42, is fighting extradition to the US, where he faces up to 70 years in prison if he is found guilty of gaining access to and damaging 97 American Navy, Army, Nasa and Pentagon computers.

His supporters held a press conference today to make a desperate plea for him to be prosecuted in the UK on a lesser charge instead.

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Cambridge University professor Simon Baron-Cohen, a leading expert on Asperger's Syndrome, said what Mr McKinnon did was "the activity of somebody with a disability rather than a criminal activity".

Mr McKinnon, from Wood Green, north London, was only diagnosed with the condition in August last year.

He insists he was looking for evidence of UFOs when he hacked into the US military networks in 2001 and 2002.

Prof Baron-Cohen said Mr McKinnon's behaviour was typical of the "social naivety" that is common in people with Asperger's Syndrome.

He said: "It can bring a sort of tunnel vision so that in their pursuit of the truth they are blind to the potential social consequences for them or for other people."

The academic added: "There are questions about whether he should be imprisoned at all because someone with Asperger's Syndrome will find it very difficult to tolerate a prison environment.

"If, as I believe, the crime was committed through naivety and through an obsession - in this case with computers and trying to find information - without any intent to deceive, without any attempt to hide what he was doing, we should be thinking about this as the activity of somebody with a disability rather than a criminal activity."

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Prof Baron-Cohen said Mr McKinnon "believed that what he was doing was right" when he hacked into the computers because he was trying to uncover the truth.

Asperger's Syndrome is a form of autism. Sufferers often struggle to communicate with other people and can develop obsessive interests.

Mark Lever, chief executive of the National Autistic Society, said: "We are calling for urgent action to stop this extradition and allow him to stand trial in the UK."

Mr McKinnon's solicitor, Karen Todner, said she had written to Foreign Secretary David Miliband asking him to appeal to George Bush to pardon her client in his final days as US president.

She has also written to Keir Starmer QC, the new director of public prosecutions, inviting him to bring charges against Mr McKinnon in the UK.

Ms Todner enclosed a statement from Mr McKinnon accepting that his hacking constituted an offence under the Computer Misuse Act 1990.

An application for permission for a judicial review of the proposed extradition will be heard at the High Court on Tuesday.

Ms Todner said: "If this fails, it will be likely that Gary will be on a plane within days with no guarantee he will ever return."

The US military alleges that Mr McKinnon caused 800,000 dollars-worth (£532,500) of damage and left 300 computers at a US Navy weapons station unusable immediately after the September 11 2001 terror attacks.

He is accused of hacking into 53 US Army computers and 26 US Navy computers, including those at US Naval Weapons Station Earle in New Jersey, which is responsible for replenishing munitions and supplies for the Atlantic fleet.

He is also accused of hacking into 16 Nasa computers, one US Department of Defence computer and one machine belonging to the US Air Force.

Mr McKinnon was caught in 2002 as he tried to download a grainy black and white photograph which he believed was an alien spacecraft from a Nasa computer housed in the Johnson Space Centre in Houston, Texas.

He was easily traced by the authorities because he used his own email address.

Mr McKinnon said he believed he would get a fairer trial in the UK.

He told the press conference in central London the strain of his situation was getting to him.

"I'm on beta-blockers at the moment, I'm extremely stressed," he said.

"I am very controlled, which is probably not a good thing, but inside the fires of hell are burning. It's not a good pace to be."