Study says British man wanted in US for hacking Pentagon computers could be very likely to attempt suicide if extradited

Hopes that the computer hacker Gary McKinnon will win his 10-year legal battle against extradition have been significantly raised after Home Office-appointed psychiatrists warned that he would be very likely to attempt suicide if sent for trial in the US.

The home secretary, Theresa May, is expected to announce her decision to parliament on Tuesday morning. McKinnon's mother, Janis Sharp, said she has been given no prior word about the likely verdict. However, the family has been shown a new government-commissioned medical report into the mental condition of McKinnon, who has Asperger's syndrome and has suffered severe depression.

The study, by Professor Declan Murphy and Professor Tom Fahy of the Institute of Psychiatry, has been seen by the Guardian and spells out the risks the government would face if it pressed ahead with McKinnon's extradition.

It is particularly significant because the same experts concluded in July that McKinnon's risk of suicide was "moderate". Since then they have studied reports from three experts in Asperger's and suicide – Professor Simon Baron-Cohen, Professor Jeremy Turk and Dr Jan Vermeulen – who examined McKinnon earlier this year.

The new report is dated 24 September. In a section headed "Opinion", Murphy and Fahy write: "It is clear from [Vermeulen's] report that there is a significant risk of suicidal behaviour and that Mr McKinnon 'will do' what he has threatened for the last three to four years if the extradition proceeds. On this specific point, we cannot offer reassurance to the authorities who are dealing with this case."

Sharp said the family was delighted at the experts' change of opinion: "I was really surprised when I read it, and so was Gary's solicitor. I didn't expect it.

"Although it's saying bad things about Gary's health, it's very good news. For them to come out and say Gary not even might or could but will take his own life if it proceeds, and they can give no reassurances, I would have thought it would be very difficult for the government to go ahead."

McKinnon, 46, was arrested in March 2002 for allegedly hacking into dozens of Pentagon and Nasa computers over a 13-month period from a bedroom in north London. He admitted the security breaches but denied any malicious intent, saying he was looking for evidence of UFOs, and that notes he left mocking the lack of security were merely mischievous. US officials insist he caused thousands of dollars' worth of damage and want to prosecute him on charges that could bring a jail sentence of 60 years.

Much of the subsequent extradition fight, which has seen McKinnon's lawyers appeal unsuccessfully to the House of Lords and the European court of human rights, has centred on his mental health and the possible risk of suicide if he is extradited.

Sharp's campaigning over the years has played a significant role, particularly as she secured guarantees of support from the Liberal Democrats while they were in opposition and, to a lesser extent, the Conservatives. David Cameron has raised the case twice in meetings with Barack Obama in a presumed attempt to smooth the diplomatic path towards an eventual decision not to extradite.

However, Sharp, from south Hertfordshire, said the delay since the new government took power had been agonising.

"Before the coalition got in Nick Clegg stood by my side outside the Home Office to support me, and David Cameron spoke up for Gary and said he should be tried here. When they won the election we wept as we thought Gary was home and dry. Then nothing happened. Then Theresa May put a stop on it and we thought, fantastic. Again nothing happened. After that, David Cameron said the decision would come in weeks, not months. Then somebody in the Home Office decided they needed more medical reports."

May's decision was due in July but was delayed until October because of the Olympics.

The Home Office declined to say when May would make her announcement, only that the high court had directed that it should be "on or around" 16 October.

A spokesman said: "This is a complex case, in a complex area of the law, and a large amount of material has been submitted, some of it relatively recently. The home secretary needs to consider all the material carefully before making a decision."

Sharp said she would go to her son's home nearby and await the decision with him. The 10-year battle had taken an enormous toll on them both, she said.

"It's the most cruel and horrendous thing, and it's the only thing in our minds 24 hours a day. It's the only thing I can talk about. I've gone through all our money. Gary hasn't got an outlet. He can't go online. He's a really good musician but he hasn't touched an instrument for years because he can't deal with what it would bring up. So he sits in the dark with his cats. He's lost 10 years of his life.

"I just want Gary to recover mentally, to get back to being himself, not seeing the world as this big, dark, cruel place. He's a good guy and it's all so sad."

Timeline

February 2002 McKinnon allegedly hacks into 97 US military and Nasa computers from north London.

19 March 2002 McKinnon is arrested by British police.

31 October 2002 The district court of New Jersey issues a warrant for his arrest.

2004 The US government files a request to have McKinnon extradited.

10 May 2006 A district judge recommends McKinnon's extradition, but leaves the final decision to the then home secretary, John Reid.

4 July Reid signs an order to extradite him to the US.

3 April 2007 McKinnon loses a high court appeal against his extradition.

30 July 2008 McKinnon loses a House of Lords appeal against his extradition.

25 August 2008 He is diagnosed with Asperger's syndrome.

28 August 2008 The European court of human rights rules it will not prevent McKinnon's extradition.

1 August 2009 McKinnon loses a further high court appeal.

13 January 2010 A high court judge says McKinnon would be a suicide risk if extradited and the home secretary may have acted unlawfully by pursuing his case.

20 May 2012 Theresa May, now home secretary, puts the extradition on hold to consider new medical evidence.

21 July 2012 After meeting Barack Obama, David Cameron, says McKinnon could instead serve time in a UK jail.

16 October 2012 May is due to tell the House of Commons of her decision.