LONDON, UK: A British computer expert wanted by the United States for "the biggest military hack of all time" begins a final attempt on Tuesday to avoid extradition.

Lawyers for Gary McKinnon, 43, will argue in London's High Court that he is too ill to be sent to the United States for trial because he has Asperger's syndrome, a form of autism.

His defence team say he would be at risk of suicide or mental illness if he were extradited, the Press Association said.

U.S. prosecutors accuse McKinnon of causing $700,000 of damage and shutting down the U.S. Army's entire network of more than 2,000 computers in Washington for 24 hours.

At the time of his indictment, Paul McNulty, U.S. Attorney for the Eastern District of Virginia, said: "McKinnon is charged with the biggest military computer hack of all time."

McKinnon says he was searching for proof of alien life when he repeatedly broke into Pentagon and NASA networks from his home computer in London in 2001 and 2002.

Using a limited 56K dial-up modem, he found many U.S. security systems used an insecure Microsoft Windows programme with no password protection. He then scanned U.S. military networks using off-the-shelf software.

The hearing is likely to end on Wednesday, though the judges may not announce their decision for some weeks, an assistant to McKinnon's lawyer Karen Todner told Reuters.

Since his arrest in 2002, McKinnon has lost repeated legal attempts to avoid extradition. He wants to be tried in London, where he would expect a more lenient sentence if convicted. A U.S. court could jail him for up to 70 years.

The European Court of Human Rights and Britain's highest court, the House of Lords, have both approved his extradition.

McKinnon's campaign has won celebrity backers, including British singer Sting, Pink Floyd's David Gilmour and former Beirut hostage Terry Waite.

"A lot of this rests on intent," Waite told Sky News. "His intent was to try and discover whether there were little green men around. Well, that's rather ridiculous, isn't it?"