When he wakes up this morning, Gary McKinnon will be 72 hours from learning whether he is on the fast track to a 60-year prison sentence, thanks to his obsession with aliens.

McKinnon, 42, from Enfield in north London, is accused by American prosecutors of illegally accessing top-secret computer systems in what they claimed in one legal document was 'the biggest military computer hack of all time'.

The self-taught IT expert insists he was simply looking for information the US government had on UFOs and is adamant that he never damaged any of its computer systems. This argument, however, cuts little ice with the Americans, who are trying to extradite him. Five years after being told by British police that he would probably get a six-month community service order for his exploits, McKinnon finds himself still wanted by the US authorities. A 2006 High Court ruling granted the extradition request, and on Wednesday the House of Lords will decide on McKinnon's appeal against that ruling.

That it should come to this is little short of outrageous, say his supporters. Soon after he was arrested in 2002, US prosecutors appeared to offer McKinnon a deal: if he agreed to extradition and admitted his guilt, he would get a sentence of three to four years, most of which could be served in the UK. When McKinnon rejected the offer - made in confidential meetings at the US embassy - his lawyers were told 'all bets were off'. They claim the US prosecutors upped the stakes, suggesting he would be 'treated like a terrorist' if he did not agree to face trial and plead guilty in the US.

McKinnon claims that at one stage there were suggestions that he would face a military tribunal, possibly at Guantánamo Bay. 'They said they wanted to see me fry,' he said.

McKinnon's lawyers claim that attempts to force him to accept a plea bargain constituted 'an unlawful abuse of the court process'.

A Lords ruling in favour of McKinnon, who has become a cause célèbre for UFO enthusiasts, computer users and civil liberties groups, would force US prosecutors to restart their extradition process in the magistrates' courts, a major setback that could have ramifications for other Britons resisting removal to the US. A ruling against him would mean an appeal to the European Court of Human Rights and leave him in legal limbo, banned from travelling abroad, forced to report to police every Friday, and barred from accessing the internet.

In a further twist, it has emerged that a crucial file containing details of the early meetings with the US prosecutors, at which the offers were apparently made, has gone missing from the office of McKinnon's solicitor. A laptop holding details of the same meetings was stolen from the car of one of his barristers.

The revelations have prompted febrile speculation among McKinnon's supporters, who fear that events have taken a sinister turn. McKinnon believes his phone has been bugged and claims to have been followed. As a result of his exploits, no IT company will now offer McKinnon a job. 'I think it's bloody ridiculous,' he said. 'They should employ me to bust paedophile rings or credit card frauds rather than stick me in jail for the rest of my life.'

These days he earns a living driving a fork-lift truck. It seems a mundane job for a man who between 1999 and 2002 broke into the most secure computer systems in the world from his north London flat. Using a computer language called Perl and a cheap PC, McKinnon linked a number of computer systems to search for US databases that were not protected by a password. 'I could scan 65,000 machines in less than nine minutes,' McKinnon said.

McKinnon unearthed unprotected computer systems operated by the US army, the navy, the Pentagon and Nasa. On every system he hacked, he left messages. 'It was frightening because they had little or no security,' he said. 'I was always leaving messages on the desktop saying, "your security is really crap".'

One message has come back to haunt him. 'I said US foreign policy was akin to government-sponsored terrorism and I believed 9/11 was an inside job. It was a political diatribe,' he admitted.

In the end, the ease with which he could hack the systems became his undoing. 'I got sloppy. I went to places directly rather than jump through systems. Nasa tracked back my IP address.'

McKinnon's interest in aliens was started by an internet-based group of UFO enthusiasts called The Disclosure Project. The group had collected more than 200 testimonies - some from people who have served in the US military - that 'confirm' that extra-terrestrials exist. Not only that but, according to McKinnon, some of the testimonies offered proof that 'certain parts of Western intelligence had acquired and reverse-engineered their technology, mainly weaponry and free energy'.

Intrigued, McKinnon used the testimonies to help him search top-secret US databases for information about free energy. 'I felt if it existed it should be publicly available,' he said. He says he came across many other hackers in the supposedly secure systems, many with Chinese and Russian internet addresses. Since his exploits were exposed, consecutive government reports have confirmed that the US military's computer systems remain poorly protected.

McKinnon was caught before he could find any confidential information on 'free energy', but he saw enough to believe the US authorities are suppressing what they know about aliens. He says he came across a document written by a Nasa official who claimed the agency has to airbrush UFOs out of satellite photos because 'there are so many of them'.

With only a 56k modem, he found that downloading the huge volume of documents was too time-consuming. But McKinnon claims that he managed to capture almost two-thirds of an image of what he believes was either a UFO or a top-secret US craft operating in space.

The picture was confiscated, along with all the other material McKinnon downloaded. The material included an Excel spreadsheet entitled 'non-terrestrial officers' and a list of names. 'It was a really weird phrase,' McKinnon said. 'Maybe it was the secret development of a space force. Space is the next frontier and it's already being weaponised.'

His hacking career came to an abrupt end one morning in March 2002. The National High Tech Crime Unit searched his flat and arrested McKinnon and his then girlfriend. 'They said "you'll probably get six months' community service",' McKinnon claimed.

In the end the Crown Prosecution Service decided not to prosecute, but two years later, after crime unit officials visited Washington, apparently taking McKinnon's hard drive, the US government began extradition proceedings. 'Now I'm facing 60 years in prison,' McKinnon said. 'I believe my case is being treated so seriously because they're scared of what I've seen. I'm living in a surreal, nutter's film.'

The greatest hackers

Jonathan James At 16 he was the first juvenile to be jailed (for six months) for hacking in 2000. He targeted high-profile organisations including Nasa, stealing more than $1.7m worth of software.

Adrian Lamo Broke into organisations such as the New York Times and Microsoft between 2002-2003 using internet connections at coffee shops and libraries. He had to pay $65,000, serve six months of home confinement and two years' probation.

Kevin Mitnick The Department of Justice called him 'the most wanted computer criminal in United States history' for his hacking activities between 1982 and 1992. He served five years, eight months in solitary confinement.

Kevin Poulsen Known as Dark Dante, he hacked into LA radio's KIIS-FM phone lines, earning himself a Porsche. Called 'Hannibal Lecter of computer crime' for his hacking activities between 1985 and 1991, he served five years in prison.

Source: itsecurity.com