Mr McKinnon's mother believes extradition will worsen his mental state
The extradition to the US of computer hacker Gary McKinnon should be halted owing to his "precarious state of mental health", MPs say.
The Home Affairs Committee also said there was a "serious lack of equality" in US-UK extradition arrangements.
But the Home Office said there was "no imbalance" and no need for a review.
Mr McKinnon, who has Asperger's syndrome, is accused of breaking into the US military computer system but says he was just seeking UFO evidence.
The Glasgow-born 43-year-old, now of Wood Green, north London, faces 60 years in prison if convicted in the US.
'Lack of equality'
The Commons committee is conducting an inquiry into the US-UK extradition treaty, which critics say does not treat American and British citizens equally.
A letter, from committee chairman Keith Vaz to Home Secretary Alan Johnson, said the MPs had received "clear, legal opinion" from two lawyers that the home secretary had greater scope to exercise his discretion in Mr McKinnon's case than the minister believed.
The letter urged Mr Johnson to "comprehensively review" the operation of US-UK extraditions and "exercise your discretion in this case".
There was a "serious lack of equality" in the way the extradition treaty deals with UK citizens compared with US citizens, the letter added.
But in response, a Home Office statement said: "There is no imbalance in the extradition arrangements between the UK and the US.
At this stage in the case the sole issue is whether extradition would, or would not, breach Mr McKinnon's human rights
"As the home secretary told the Home Affairs Select Committee on Tuesday, the evidence that must be provided for a US extradition request to proceed in the UK is in practice the same as for a UK request to proceed in the US."
It added: "The suggestion that the operation of the Extradition Act needs to be reviewed comprehensively is unnecessary."
The home secretary is in the process of looking at new medical evidence on Mr McKinnon, which Mr Johnson has said he will consider "very carefully" before approving extradition.
He has also said he wants to give Mr McKinnon's lawyers time to examine medical reports and make legal representations to the European Court of Human Rights in Strasbourg.
But the Home Office statement pointed out that the home secretary had "no general discretion to refuse extradition".
"At this stage in the case the sole issue is whether extradition would, or would not, breach Mr McKinnon's human rights," it said.
"Unless the evidence shows that extradition would breach the European Convention on Human Rights it would be unlawful to refuse extradition."
Mr McKinnon's mother, Janis Sharp, has said her son "would rather be dead than extradited".
Appearing before the Home Affairs Committee earlier in the week, she said: "We were told this treaty was to be used mainly for terrorists.
"People like Gary are not terrorists. We should stand up to America and say, 'This is wrong.'
"This has ruined Gary's life. It's ruined our lives," she said.
Mr Johnson responded by insisting that the US had a "proper, mature legal system".
"It's almost as if you are talking about an enemy state," he said.
"In a world of international crime where criminals cross borders much more frequently, then you need to have proper arrangements in place.
"Extradition is not a statement of guilt or innocence. It's quite proper for the US to ask to bring over people who have committed crimes against their country."