Lawyers acting for Gary McKinnon say the self-confessed Nasa hacker runs the risk of becoming psychotic and suicidal if his extradition to the US goes ahead.
Edward Fitzgerald, QC, described the risk during a hearing on Tuesday at the Royal Courts of Justice in London. Judges Lord Justice Stanley Burton and Mr Justice Wilkie are reviewing a decision by former home secretary Jacqui Smith to allow extradition proceedings against McKinnon to go ahead, despite his being diagnosed with Asperger's syndrome.
"There is a risk of psychotic disruption, which may range on a path from anxiety through to psychosis," Fitzgerald told the court, as he presented arguments against the extradition. "There is a risk [McKinnon] may take his own life."
Fitzgerald said the home secretary reached a flawed decision in response to the medical evidence. "She underestimated and misrepresented the gravity of the situation."
Fitzgerald went on to point out that Smith had not asked the US authorities to repatriate McKinnon should he be found guilty by a US court.
McKinnon has been accused by US prosecutors of hacking into US military systems between February 2001 and March 2002, using his home computer in North London. They allege that McKinnon accessed 97 US government computers, including US army, navy and Nasa computers responsible for national defence and security, and naval munitions supply.
Moreover, the US authorities claim McKinnon deleted critical operating-system files, leading to the shutdown of the entire US army network of over 2,000 computers in the District of Washington. The deletion also took down a US Naval Weapons Station computer system, causing $700,000 (£430,000) damage, prosecutors allege.
McKinnon has admitted deleting logs in an attempt to cover his movements, but has denied causing any damage. He claims to have been hunting for evidence of UFOs.
"The issue of damage has been overblown all along," said Fitzgerald. "[McKinnon] does not accept he deleted materials, aside from his own."
On Tuesday morning, the judges expressed their intention to reserve judgement, which means their decision might not be made public for up to two weeks.
McKinnon's solicitor, Karen Todner, told ZDNet UK that the court's decision could go a number of ways other than in favour of the extradition as it stands. For example, the judges may say they will not order the extradition until the home secretary has asked for, and received, formal assurances that McKinnon will serve any US prison sentence in the UK. Or the judges may decide against extradition, in which case McKinnon may still be prosecuted in the UK.
If the extradition is given the go-ahead, then the defence will attempt to appeal to the House of Lords, Todner said. In addition, the defence will seek a judicial review of the decision by the Crown Prosecution Service (CPS) not to prosecute McKinnon in the UK.
McKinnon's mother, Janis Sharp, told ZDNet UK that McKinnon's health had suffered as a result of the stress of the trial.
"He's just had an operation on his eye, he had a lump removed and sent for biopsy," said Sharp. " There's a lump growing on his shoulder. It's the stress — he's stressed out of his mind."