A Briton accused of hacking into NASA computers should not be extradited to the US due to his mental health, a lawyer said on Tuesday, adding that he was eccentric and "passionate" about UFOs.

Gary McKinnon, 42, could spend life in prison if convicted by a US court of gaining access to 97 computers in 2001 and 2002 in the aftermath of the September 11, 2001 attacks.

McKinnon says he was only looking for evidence of unidentified flying objects (UFOs) when he hacked into the US Navy and NASA space agency computers.

And his lawyers said on Tuesday that McKinnon - who suffers from Asperger's Syndrome, a form of autism - could commit suicide or suffer psychosis if extradited and could easily be prosecuted in Britain.

McKinnon was "an eccentric person who has passionate views about UFOs", his lawyer Edward Fitzgerald told the High Court in London, adding that extradition was "unnecessary, avoidable and disproportionate".

McKinnon's lawyers had hoped he could be prosecuted in Britain and face a less severe sentence, but the Crown Prosecution Service (CPS) ruled in February that the case was best brought in the United States.

They are seeking a judicial review of a decision by then home secretary Jacqui Smith last October to order his extradition.

Fitzgerald said the then minister had not realised the severity of McKinnon's mental problems.

"She underestimated the gravity of the situation without obtaining evidence of her own," he said.

British prosecutors have declined to begin proceedings against McKinnon, pending the US extradition request. The hearing continues on Wednesday.

AFP