The mother of computer hacker Gary McKinnon made an impassioned appeal to US president Barack Obama today after her son failed in his latest High Court bid to avoid extradition to America.

The 43-year-old, who suffers from Asperger's Syndrome (AS), is wanted for trial on charges of hacking into US military networks.

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His mother, Janis Sharp, has expressed fears that he could face a 60-year sentence in a tough US jail and would be at real risk of suicide because of his medical condition. She also fears she would never see him again.

Today Lord Justice Stanley Burnton and Mr Justice Wilkie ruled extradition was "a lawful and proportionate response" to Mr McKinnon's offending, and said the Home Secretary was entitled to accept US assurances that he would receive "appropriate care" in America.

After the judgment, Home Secretary Alan Johnson made clear that he has no plans to stop Mr McKinnon's removal.

He said: "It would be illegal for me to stop the extradition of Gary McKinnon, which the court ruling has made clear.

"Mr McKinnon is accused of serious crimes and the US has a lawful right to seek his extradition, as we do when we wish to prosecute people who break our laws.

"He is accused of hacking into 97 US Army, Navy, Nasa and Defense Department computers concerned with national defence and security at a critical time immediately following the 9/11 attacks and leaving the military network vulnerable to intruders.

"The court judgment has also made absolutely clear that the DPP's decision not to prosecute in the UK was the right one.

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"My predecessor has already sought and received clear assurances from the US that Mr McKinnon's health and welfare needs would be met, should he be extradited."

But immediately after the "heartbreaking" High Court ruling, Mrs Sharp made a dramatic, desperate appeal to Mr Obama to halt her son's removal as she stood on the steps of the Royal Courts of Justice in London.

Directly addressing Mr Obama, she said: "Stand by us and make this world a better place, a more compassionate place."

She added: "Obama wouldn't have this. He doesn't want the first guy extradited for computer misuse to be a guy with Asperger's, a UFO guy. He wouldn't want this.

"I'm just praying, please hear us, Obama, because I know you would do the right thing.

"I know you would have the strength to stand up and not have this."

Mr McKinnon's solicitor, Karen Todner, said she would lodge an appeal against the High Court's decision within 28 days and, if possible, take it to the UK's new Supreme Court and even to Europe.

A letter has been sent to the US president signed by 40 British MPs asking him to step in and "bring this shameful episode to an end".

Lawyers for Mr McKinnon, who was told the decision yesterday, described him as a "UFO eccentric" who had been searching for evidence of extra-terrestrial life, and described the idea that he was a danger to US national security as "a complete fantasy".

The US authorities said Mr McKinnon was responsible for the "biggest military hack of all time" that had been highly damaging.

A campaign involving family, politicians, civil rights groups, sympathetic media and celebrities has supported the hacker's long battle against extradition.