London: The lawyers for computer hacker Gary McKinnon are to challenge the home secretary`s decision not to block his extradition to the US.
They said they would fight on after Alan Johnson told Mr McKinnon`s family medical grounds could not prevent it.
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.
His lawyer, Karen Todner, said: "It`s a devastating blow but we are not going to give up. We are certainly coming to the end of the road.
"We`re just hoping at some point someone sees sense and steps in. All the legal team do know is we cannot give up because in some ways it`s like dealing with a death row case, and we genuinely believe that Gary`s life is at stake here."
She said she would issue judicial review proceedings next week - a process she said she was given just seven days to complete, rather than the more normal three months.
If that failed, they would take the case to the European Court of Human Rights, she added.
Johnson said he had carefully considered the representations but concluded that sending Mr McKinnon to the US would not breach his human rights.
"Due to legitimate concerns over McKinnon`s health, we have sought and received assurances from the United States authorities that his needs will be met," he said.
But Ms Todner said he had gone against independent legal advice which said he could have used his discretion.
McKinnon admits hacking into 97 US government computers, including Nasa`s and the Pentagon`s, during 2001 and 2002.
He has told media 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, who has a form of autism, 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.
Liberal Democrat home affairs spokesman Chris Huhne said it was appalling the government placed a higher value on a "deeply unfair" extradition agreement than on the welfare of a British citizen.
"The home secretary should stop being an American poodle and start being a British bulldog," he said.
And Shami Chakrabarti, director of Liberty, said: "The shoddy treatment of this vulnerable man should demonstrate that our rotten extradition laws need urgent reform."
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.
McKinnon has been the focus of a campaign to prevent his removal to the US.
Earlier, the Commons` Home Affairs Committee said the move should be halted owing to his "precarious state of mental health".
They said there was a "serious lack of equality" in the way the extradition treaty deals with UK citizens compared with US citizens.