Computer hacker Gary McKinnon is facing Christmas in a US jail after the Home Secretary refused to block his extradition.
Alan Johnson finally dashed hopes that the government would intervene in the case, insisting there was no evidence that the extradition would breach Mr McKinnon's human rights.
Mr McKinnon's mother Janis Sharp said yesterday that ministers should "hang their heads in shame" for the "barbaric" decision.
His legal team is to apply for a judicial review within days, but Ms Sharp warned that, if that was not granted, then her son could be extradited by Christmas.
Mr McKinnon, who has Asperger's syndrome, faces up to 60 years in jail for hacking into Pentagon computers soon after the September 11 2001 terror attacks. He claims he was looking for evidence of UFOs.
Mr McKinnon has been refused leave to appeal to the Supreme Court, so a judicial review is the last remaining option open to him.
Mr Johnson had previously said he had "stopped the clock" on the deportation after receiving new medical evidence, but on Thursday night he declared: "I am clear that the information is not materially different from that placed before the High Court earlier this year and does not demonstrate that sending Mr McKinnon to the United States would breach his human rights."
Mr McKinnon's family say that his health has deteriorated significantly under the pressure of the pending extradition.
Ms Sharp warned that "Gary is at risk of suicide - I'm extremely worried about him.
"To force a peaceful, vulnerable, misguided UFO fanatic like Gary thousands of miles away from his much-needed support network is barbaric. This is a cruel and miserable decision.
"If the severity of Gary's medical condition isn't sufficient to prevent his extradition, I can't imagine what is.
"God help others facing a similar fate. I'm so upset and angry."
Mr McKinnon's lawyer Karen Todner said that she planned to issue judicial review proceedings next week.
"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," she said.
Liberal Democrat home affairs spokesman Chris Huhne said: "It is appalling that this government places a higher value on a deeply unfair extradition agreement than it does on the welfare of a British citizen.
"Alan Johnson has shamefully turned down the opportunity to demonstrate his faith in British justice and save a vulnerable man from a lifetime in prison or worse."
Campaign group Liberty director of civil rights Shami Chakrabarti said: "A government can't tie its own hands and then protest that its hands are tied.
"The shoddy treatment of this vulnerable man should demonstrate that our rotten extradition laws need urgent reform."
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