Supporters make the point that Gary McKinnon has Asperger's syndrome
Computer hacker Gary McKinnon faces being tried in the US after requests to block his extradition were refused, the Home Office has confirmed.
Home Secretary Alan Johnson told Mr McKinnon's family he could not block the move on medical grounds.
Glasgow-born Mr McKinnon, 43, who has Asperger's syndrome, is accused of breaking into US military computers. He says he was seeking UFO evidence.
Now of Wood Green, north London, he faces 60 years in prison if convicted.
Mr Johnson said he had carefully considered the representations but had concluded that sending Mr McKinnon to the US would not breach his human rights.
As such, he had no discretion to block the extradition.
"Due to legitimate concerns over Mr McKinnon's health, we have sought and received assurances from the United States authorities that his needs will be met," he said.
Mr McKinnon admits hacking into 97 US government computers, including Nasa's and Pentagon's, during 2001 and 2002.
He has told the BBC he was on a "moral crusade" to prove US intelligence had found an alien craft run on clean fuel.
His mother Janis Sharp told the BBC she was "devastated" by the news and that her son had reacted "very badly".
"It's a disgusting decision. Gary has been in a heightened state of terror for almost eight years.
"To force a peaceful, vulnerable, misguided UFO fanatic like Gary thousands of miles away from his much-needed support network is barbaric," she said.
She said she was not comforted by the home secretary's advice that her son would not be held in a "supermax" jail, which hold the highest-security prisoners.
Their solicitor Karen Todner said they had seven days to put a case for judicial review and that she hoped that would be heard before Christmas.
If that failed, they would take the case to the European Court of Human Rights, she added.
Mr Johnson had last month agreed to study new medical evidence before deciding on the extradition. The High Court had previously refused permission to appeal to the Supreme Court.
Mr McKinnon has been the focus of a campaign to prevent his removal to the US.
Earlier this month, the Commons' Home Affairs Committee said the move should be halted owing to his "precarious state of mental health".
They concluded there was a "serious lack of equality" in the way the extradition treaty deals with UK citizens compared with US citizens.