A British “UFO eccentric” accused of hacking into US military networks brought a last challenge in the High Court yesterday against extradition to America on health grounds.

Lawyers for Gary McKinnon say that he should be tried in Britain because of his Asperger syndrome and the severe mental distress he could suffer if sent to the US for trial.

His lawyer, Edward Fitzgerald, QC, told the court that there was “clear, uncontradicted expert evidence” that the stress of extradition could result in psychosis and suicide because of his illness.

Mr McKinnon, from Wood Green, North London, is seeking a judicial review of the then Home Secretary Jacqui Smith’s decision last October to allow extradition.

Mr McKinnon’s supporters say he acted through naivety as a result of Asperger syndrome — a form of autism that leads to obsessive behaviour — and should not be considered a criminal.

Mr Fitzgerald accused the Home Secretary of reaching a “flawed” decision in the light of uncontradicted medical evidence of the severe mental suffering extradition could trigger.

He told Lord Justice Stanley Burnton and Mr Justice Wilkie: “She underestimated the gravity of the situation without obtaining evidence of her own. She is simply not addressing the issue.”

The US government says Mr McKinnon was responsible for the “biggest military hack of all time”, involving 97 government computers belonging to organisations including the US Navy and Nasa.

Mr McKinnon has admitted hacking into the system in 2001-02, but claims that he was looking for evidence of extra-terrestrial life.

The US government alleges that his conduct was intentional and calculated to influence and affect it by “intimidation and coercion”. It says the cost of repair was more than $700,000 (£436,000).

At the High Court in London Mr Fitzgerald described Mr McKinnon as “an eccentric person who has passionate views about UFOs”.

Extradition was “unnecessary, avoidable and disproportionate” as Mr McKinnon could be prosecuted in England, he added.

The Director of Public Prosecutions had declined to prosecute in light of the US extradition request, but if extradition did not take place, prosecution would be inevitable, the QC said.

Being forced to travel to America and face separation from his partner and family would impact on his Asperger syndrome and expose him to a high risk of serious mental deterioration, Mr Fitzgerald said.

Home Office lawyers argue that extradition is justified and would not be disproportionate, given the very serious charges Mr McKinnon faces.

The US authorities had given assurance that he would be provided with appropriate care and treatment.

Mr McKinnon, who began writing his own software programs at 14, had Asperger syndrome diagnosed last August. His lawyers have fought a series of battles to block his removal.

They lost every one until earlier this year when two senior judges ruled the fresh evidence about his health “merits substantive consideration”, leading to yesterday’s hearing.

Previously his case had been rejected on other grounds by a district judge, the High Court and then, in July last year, the House of Lords. The European Court of Human Rights in Strasbourg also refused to intervene.

Mr McKinnon was caught as he tried to download a grainy black and white photograph which he believed was an alien spacecraft from a Nasa computer in the Johnson Space Centre in Houston, Texas. He was easily traced by the authorities because he used his own e-mail address.

He has always said he had no malicious intent but was looking for classified documents on UFOs which he believed the US authorities had suppressed. He has signed a statement accepting that his hacking constituted an offence under Britain’s Computer Misuse Act 1990.

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