London - A British computer hacker accused of breaking into top secret US defence and aerospace systems Wednesday lost his appeal against extradition to the US where he could face a life sentence if convicted of sabotaging vital defence systems.

Gary McKinnon, a 42-year-old computer and UFO enthusiast from Glasgow, Scotland, has been described as the "biggest military hack of all time."

His appeal against a 2006 High Court ruling was rejected by the Law Lords Wednesday, Britain's top appeal court.

McKinnon was never charged in Britain after admitting that he accessed 97 computers of the Pentagon and the US space agency NASA between 2001 and 2002 in an alleged search for information on UFOs.

His lawyers had argued that McKinnon, who lives in London, was motivated by curiosity and that he managed to access the networks because of lax security.

US authorities claim he stole 950 passwords and deleted files.

McKinnon has been warned by the US authorities that he faced a life sentence rather than a couple of years in jail unless he agreed to plead guilty and accept extradition.

Without cooperation, said the authorities, the case could be treated as one of terrorism.

A statement by his solicitor said: "Gary McKinnon is neither a terrorist nor a terrorist sympathiser... We believe that the British government declined to prosecute him to enable the US government to make an example of him."

An appeal would be made to the European Court of Justice to prevent his removal from Britain. (dpa)