If McKinnon's appeal is rejected by the House of Lords, the former hacker faces a possible life sentence.
The indictment accuses McKinnon of perpetrating "the biggest military computer hack of all time" from his bedroom in Wood Green, London.
He is alleged to have gained unauthorised remote access to numerous US-based computers, including those of Nasa, the US Department of Defense and the US Air Force, Army and Navy.
McKinnon is fighting extradition on the grounds that, although he admits accessing some the computers, he was not a threat to security and was looking only for information about UFOs, antigravity and free energy technology.
Lawyers acting for McKinnon insist that he has become a convenient scapegoat for appalling lapses in US military security.
McKinnon says that he broke into the Pentagon's computers using a simple script that looked for access ports which retained manufacturers' default passwords.
McKinnon, also known by his online name 'Solo', admitted to an audience at security conference Infosec in 2006 that he was discovered only because he misjudged time zone differences and started using remote control software on a PC when its user was sitting in front of it.
He portrays himself as a "bumbling hacker".
US prosecutors claim that McKinnon disabled a network of 2,000 US Army computers based in Washington for 24 hours and, shortly after 11 September 2001, shut down 300 computers belonging to a US Navy weapons station.
In US judicial style, prosecutors have tried to strike a plea bargain with McKinnon. If he pleads guilty to two of the charges, he will receive a sentence of four years or less.
If he continues to protest his innocence he could face up to 70 years, according to his solicitor.
McKinnon was first arrested in 2002 by the UK's National High-Tech Crime Unit under the Computer Misuse Act, but the Crown Prosecution Service refused to charge him. Later that year he was indicted by US prosecutors.
It was not until 2005, when the UK and US signed a new extradition treaty, that the US began proceedings against him.
McKinnon then became subject to bail conditions. He was not allowed access to the internet via a computer, and had to check in to his local police station every day.
He has never been charged in the UK.
A decision from the Lords is expected in three weeks.
McKinnon makes last stand against extradition
By Andrew Charlesworth on Jun 17, 2008 8:46AM
Gary McKinnon, the UK systems administrator who hacked into Nasa and Pentagon computers in 2001 and 2002, is today making a last-ditch bid to avoid extradition to the US..
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