If Gary McKinnon loses this appeal, he would be the first British hacker extradited to the U.S. He could face up to 60 years in prison.

Gary McKinnon told the highest court in the land that he was the victim of an oppressive prosecution by US authorities who had abused British law by trying to force him into a plea bargain.

Mr McKinnon, 44, a systems analyst, is accused of causing $475,000 worth of damage by gaining access to 97 computer systems belonging to the Pentagon, Nasa and the US military.

He claims he was only looking for evidence that America had concealed the existence of UFOs.

One of his attacks happened immediately after Sept 11 and targeted the Earle Naval Weapons Station in New Jersey, shutting down its computer system for a week.

Mr McKinnon was caught after some of the software used in the attacks was traced back to his girlfriend's e-mail account.

His lawyer David Pannick QC told five Law Lords that it would be an abuse of extradition proceedings to send Mr McKinnon to the US because of the way US authorities had behaved.

Mr McKinnon had been warned by the US that, unless he agreed to plead guilty, he faced a life sentence rather than a couple of years in jail, Mr Pannick said.

The threats from US authorities also included one from New Jersey prosecutors that Mr McKinnon "would fry", said Mr Pannick.

If Mr McKinnon refused a plea bargain then the matter could be treated as a terrorism case which could result in a 60-year sentence in a US maximum security prison if he was found guilty, the Law Lords were told.

Mr McKinnon, also known as "Solo", is alleged to have carried out the hacking from his home in north London. He was arrested in 2002 but never charged in the UK.

Later that year the US government indicted him and began extradition proceedings in 2005.

Former Home Secretary John Reid granted the US request and Mr McKinnon began his court battle.

Last year his lawyers argued at the High Court in London that he had been subjected to "improper threats" and the extradition would breach his human rights but two judges said they could find no grounds for appeal.

Mr McKinnon has never denied that he accessed the computer networks of a wide number of US military institutions between February 2001 and March 2002.

He has always maintained that he was motivated by curiosity and that he only managed to get into the networks because of lax security.

Mr Pannick told the Law Lords: "It is not in dispute that the courts enjoy a power to refuse to extradite alleged offenders under the Extradition Act if the court considers there has been an abuse of process.

"The US prosecutors sought to impose pressure on the appellant, through his legal advisors, to consent to extradition and plead guilty."

Mr McKinnon was told if he co-operated he would receive a lesser sentence of between 37 to 46 months and be repatriated to the UK where he would enjoy the benefits of the UK parole system, Mr Pannick said.

He said: "By contrast, the appellants representatives were told that if he declined to co-operate this sentence would be in the region of eight to ten years, possibly longer. This was pressure of an unacceptable degree."

Mr Pannick also said the US authorities had indicated that charges involving "significantly damaging national security" would not be pursued if McKinnon co-operated.

Clare Montgomery QC, representing the Home Secretary, is arguing that no threats were made against Mr McKinnon by the US authorities and the extradition should therefore go ahead.

A judgment is expected within three weeks.

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