A British man who allegedly hacked into the Pentagon's computer network should not be extradited to America because US prosecutors tried to intimidate him into cooperating with them, the House of Lords was told today.

Gary McKinnon, a systems analyst who faces up to 60 years in jail if his extradition goes ahead, was the victim of an abuse of process, the panel of five law lords was told.

The 44-year-old is accused of causing £475,000 in damage by hacking into 97 computer systems operated by the Pentagon, Nasa and elsewhere in the US military.

McKinnon, a self-taught hacker who operated under the name Solo, admits accessing the computers but insists he only did so to try to find evidence for UFO landings, and that he was only able to gain access because of lax security.

His activities, described by British prosecutors in 2005 as the "biggest ever military computer hack", were carried out using a low-speed dialup connection from a room in his girlfriend's aunt's house in Crouch End, north London.

McKinnon was first arrested in 2002 but has never been charged in the UK. Last year, he lost a high court appeal against a government decision to grant his extradition to the US.

Addressing the Lords today, his lawyer, David Pannick QC, said McKinnon was told by US authorities that unless he agreed to plead guilty and to extradition he faced a much heavier jail sentence.

A US embassy legal official involved in the case had also quoted New Jersey authorities as saying they wanted to see McKinnon "fry", said Pannick, an apparent reference to the electric chair.

It was "not in dispute" that a UK court had the power to turn down an extradition if it was thought there had been abuse of process, Pannick told the law lords.

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

McKinnon was told that if he cooperated he would receive a jail sentence of between 37 and 46 months and would be repatriated to the UK for possible parole after half this time, the lawyer said.

"By contrast, the appellant's representatives were told that if the appellant declined to cooperate, this sentence would be in the region of eight to 10 years, possibly longer," Pannick said, adding that one threat was to treat the matter as a terrorism case, resulting in a possible 60-year term.

US officials additionally said any transfer to the UK would also be ruled out if McKinnon fought extradition, Pannick told the law lords.

"This was pressure of an unacceptable degree," he said.

A judgment is expected within three weeks.