The government has promised to help ensure the welfare of a computer hacker with a form of autism who faces extradition to the US to stand trial.
Gary McKinnon, 43, who has Asperger's Syndrome, is accused of the biggest ever military computer hack in 2001/02.
Labour's deputy leader Harriet Harman told the BBC it would be illegal to intervene over his extradition.
But the government would push for him to serve any prison sentence in the UK if he was convicted, she said.
Ms Harman said: "There certainly have been assurances sought and given that if, and when, the extradition takes place... his health needs will be attended to."
Mr McKinnon, from north London, and his supporters have argued he should not be extradited because of his disorder - a form of autism. He maintains he was only ever seeking UFO evidence.
Harriet Harman: "Gary McKinnon should be properly looked after when he's over there"
Home Secretary Alan Johnson has said he would be breaking the law if he blocked Mr McKinnon's extradition.
He said he could only prevent extradition in very specific cases such as when a death sentence could be involved or if the person had already been extradited to the UK from elsewhere.
But Mr McKinnon's mother Janis Sharp, who criticised the government for not protecting her son's rights, said Lord Carlile had told the home secretary he could stop the extradition.
Mr Johnson's predecessor Jacqui Smith formally gave the extradition the go ahead in October 2008.
Writing in the Sunday Times, Mr Johnson acknowledged it was "understandable" that many would be sympathetic to "someone who appears to be a misguided, vulnerable young man".
But he added "the crimes he is accused of are far from trivial" and said Mr McKinnon "should be tried fairly for them in a court of law and in the country where the impact of those crimes were felt".
The home secretary also denied extradition law was wrong, arguing it was appropriate for "an age where crime is increasingly indifferent to national borders".
Glasgow-born Mr McKinnon could face 60 years or more in prison if convicted in the US.
He admits hacking by accessing 97 government computers belonging to organisations such as the US Navy and Nasa, but denies it was malicious. He also denies the allegation he caused damage costing $800,000 (£487,000).
Mr McKinnon has always insisted he was looking for classified documents on UFOs, which he believed the US authorities had suppressed.
He has challenged refusals by the home secretary and the director of public prosecutions (DPP) to try him in the UK.
But the DPP refused to order a UK trial, saying the bulk of the evidence was located in the US and Mr McKinnon's actions were directed against the US military infrastructure.She also said it was not the place of ministers to intervene in the justice system.
And two judges rejected his court bid to avoid extradition, ruling that it was "a lawful and proportionate response" to his offence, even though they conceded he might find extradition and prison in the US "very difficult indeed".
Mr McKinnon has already appealed unsuccessfully to the House of Lords and the European Court of Human Rights.
But the case has led to a political row, with Tory leader David Cameron saying it raised "serious questions" about the extradition pact between the US and UK.
Liberal Democrat home affairs spokesman Chris Huhne has argued the American government would not "hang one of their citizens out to dry in the same way".
A letter has been sent to US President Obama signed by 40 British MPs asking him to step in and "bring this shameful episode to an end". Ms Sharp has also called on the president to intervene.
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