Gary McKinnon, a Briton with Asperger's Syndrome, became an icon for legal campaigners after battling for a decade against his extradition to the United States for hacking into military computers.

On Tuesday, interior minister Theresa May finally said Britain would block the 46-year-old's extradition -- sparing him up to 60 years in jail for what US prosecutors have called "the biggest military computer hack of all time".

McKinnon was arrested at his flat in London in 2002 after breaching dozens of Pentagon and NASA computers, leaving 300 machines at a naval air station unusable just after the terrorist attacks on September 11, 2001.

Gangly, with gaunt features and a mop of curly brown hair, McKinnon was not surprised to find officers from Britain's hi-tech crime unit at his door.

He had made little effort to cover his digital tracks, even using his own email address when breaking into US government systems.

"I almost wanted to be caught, because it was ruining me," he told the BBC.

"I'd stopped washing at one point. I wasn't looking after myself. I wasn't eating properly. I was sitting around the house in my dressing gown, doing this all night."

McKinnon has never denied the hacking, which the US says caused $800,000 (615,000 euros) worth of damage.

But he denies his intentions were malicious -- he says he was simply looking for classified documents about UFOs.

"I was on a moral crusade," he told the BBC in 2009.

"I was convinced, and there was good evidence to show, that certain secretive parts of the American government intelligence agencies did have access to crashed extra-terrestrial technology which could, in these days, save us in the form of a free, clean, pollution-free energy.

"I didn't think about jail sentences at the time," he added.

Born in the Scottish city of Glasgow, McKinnon left school at 17 and became a hairdresser, before gaining a qualification in computing in the early 1990s.

He began working in the industry and by the end of the decade was using his hacking skills to pursue his growing obsession with alien spacecraft.

McKinnon used commercially-available software to explore the US military computing system, and found many machines lacking adequate protection.

He apparently left cyber notes warning the US government of their poor security -- as well as a message comparing US foreign policy to state-sponsored terrorism.

"To a young man with Asperger's syndrome, telling the truth as he saw it was as natural as getting out of bed in the morning," McKinnon's mother Janis Sharp said in an open letter to May and Prime Minister David Cameron in September.

"His crime was tapping on a keyboard in his bedroom in north London in search of information on aliens from outer space," added Sharp, who has campaigned vigorously for her son over the last decade.

On Tuesday, she thanked May "from the bottom of my heart" for allowing McKinnon to stay in Britain on human rights grounds, after psychiatrists warned there was a high risk of him attempting suicide in the United States.

McKinnon was diagnosed with Asperger's, a form of autism, in 2007, after an autism expert watched him in a television interview and contacted McKinnon's lawyer.

Asperger's sufferers often become obsessed with certain activities or interests, as well as having difficulties in social interaction.

"Mr McKinnon is accused of serious crimes, but there is also no doubt that he is seriously ill," May told parliament as she announced the decision.

After a ten-year saga that has seen McKinnon lose appeals in both Britain's House of Lords and the European Court of Human Rights, the hacker has become a symbol for campaigners seeking to overhaul the British-US extradition treaty.