Gary McKinnon, the North London hacker accused of 'the biggest military hack of all time', has lost his High Court fight against extradition to the US.

McKinnon, 41, faces a prison sentence in the region of 45 years if found guilty of gaining access to 97 US military and Nasa computers, according to Edmund Lawson QC.

During a period of 18 months, McKinnon is said to have caused £370,000 worth of damage to US government machines, but claims the damage was not intentional and he was simply looking for evidence of UFOs.

McKinnon has never denied that he accessed the computer networks of a wide number of US military institutions between February 2001 and March 2002 but considered his hacking a 'game' and claims he left multiple notes on the desktops of US officials to alert them to the security threat.

'My intention was not to disrupt security. The fact that I logged on and there were no passwords means there was no security,' he said.

Graham Cluley, senior technology consultant at security vendor Sophos, said: 'This decision will doubtless send shockwaves through the hacking community, but irrespective of McKinnon's motivations, computer hacking is illegal both in the UK and the US, and it is high time people started thinking twice before engaging in such activities.'