A Scot accused of carrying out the "biggest military computer hack of all time" yesterday launched a High Court fight to stop his extradition to the United States.

Gary McKinnon, 41, originally from Glasgow, is wanted by US authorities for allegedly breaking into 100 Pentagon and Nasa computers in a bid to find evidence of UFOs.

Home Secretary John Reid last year granted the US request to extradite him for trial.

Mr McKinnon's lawyers yesterday argued that he had been subjected to "improper threats" and warned that the self-confessed hacker could face up to 60 years in jail.

Supporters said McKinnon was being made a "scapegoat" for the shortcomings of security policies on US military networks.

Edmund Lawson, QC, yesterday told two judges at the High Court in London that US authorities had made threats during the plea bargaining attempt in 2003 which infringed his family and private life rights under Article 8 of the European Convention on Human Rights.

Mr Lawson said: "It was indicated that if he didn't co-operate there would be no repatriation because that could be effectively blocked by the prosecutor.

"That, we contend, was an improper threat that could engage this court's abuse of process jurisdiction."

Mr McKinnon has never denied he wandered around the computer networks of a wide number of US military institutions between February 2001 and March 2002. He was arrested in November 2002.

But he has maintained he was motivated by curiosity and only got into networks because of lax security.

Mr McKinnon has claimed that, while logged on to a Nasa computer, he saw - but was unable to save - a picture of a cigar-shaped UFO in orbit.