Computer hacker Gary McKinnon, who has Asperger's syndrome, is at serious risk of suicide, relatives said today, after the home secretary rejected a last-ditch attempt to prevent his extradition to the US.

In a letter today Alan Johnson ordered McKinnon's removal to the US on charges of breaching US military and Nasa computers, despite claims by his lawyers that extradition would make the 43-year-old's death "virtually certain".

"The secretary of state is of the firm view that McKinnon's extradition would not be incompatible with his [human] rights," said the letter, dated 26 November. "His extradition to the United States must proceed forthwith."

The decision, described by lawyers as "callous", has prompted fresh fears about McKinnon's well-being. The letter rejected new expert medical evidence that McKinnon's health had deteriorated dramatically since losing his case in the high court in July, and meant that extradition would violate his right to life.

"Gary is at risk of suicide, I'm extremely worried about him," said McKinnon's mother, Janis Sharp. "This government is terrified of speaking up to America, and now they are allowing vulnerable people to be pursued for non-violent crime when they should be going after terrorists."

Mckinnon's lawyer, Karen Todner, said she planned to start a judicial review of the home secretary's decision. "We cannot give up because in some ways it's like dealing with a death row case, and we genuinely believe Gary's life is at stake here," she said.

McKinnon, from north London, was accused in 2002 of using his home computer to hack into 97 American military and Nasa computers, causing damage that the US government claims will cost more than $700,000 (£425,000) to repair.

This year the high court rejected arguments that the extradition would violate his rights, after lawyers argued the prospect of up to 60 years in a "supermax" jail would cause mental harm because he has Asperger's and a depressive illness.

Johnson insisted that he had received a guarantee from the US government that McKinnon would be assessed by doctors and psychologists were he transferred to prison there, and that he would receive "appropriate medical care and treatment", including counselling and medication. The assurances were given in a letter this February from the US department of justice, which was seen by the court.

The home secretary also said that should McKinnon be convicted in the US and seek to serve his sentence in Britain, the UK government would "progress his application at the very earliest opportunity". Lawyers had also argued that McKinnon could be prosecuted in the UK on lesser charges of computer misuse, preventing his extradition. "The CPS wanted to prosecute Gary, but they were told from the very top to stand aside and let America take him," said Sharp.

Critics say an extradition treaty designed to speed up extradition between the two countries works in favour of Americans and fails to adequately protect Britons.

Geoffrey Robertson QC, one of Britain's most eminent human rights lawyers, told the Guardian: "To send a British citizen to the US, without any right to bail, to face 10 years in prison for a crime for which he would be unlikely to receive any custodial sentence if tried here amounts to 'cruel and unusual' punishment' in breach of our 1689 Bill of Rights. The home secretary should not hide behind the weasel words of the European Convention when he should be following the law laid down by our own historic bill of rights."

McKinnon's legal team had hoped to join his case to that of Ian Norris, a retired business chief facing extradition to the US on charges of obstructing justice through his alleged role in an illegal cartel. His case will be heard in the supreme court on Monday. It was argued unsuccessfully that McKinnon's case raised similar legal issues and should be considered by the supreme court panel. Attempts to be heard in the supreme court failed however, although McKinnon'sHis legal team said they would be seeking a judicial review of yesterday's decision.

Sharp said: "We thought with Obama it would be different. Now the first person in the world to be extradited to the US for computer misuse is going to be a guy with Asperger's."