LONDON (Reuters) - British prosecutors said on Thursday they would not bring charges against a computer expert accused by a U.S. attorney of the “biggest military hack of all time,” dealing a blow to his bid to avoid extradition.

Computer expert Gary McKinnon poses after arriving at the High Court, in London, January 20, 2009. REUTERS/Andrew Winning

Gary McKinnon was arrested by British police in 2002 after U.S. prosecutors charged him with illegally accessing computers, including the Pentagon, U.S. army, navy and NASA systems, and causing $700,000 (489,000 pounds) worth of damage.

A British court ruled in 2006 that he should be extradited to the United States to face trial. If convicted by a U.S. court, he could face up to 70 years in prison. McKinnon has been battling the court decision ever since.

His lawyers asked the Director of Public Prosecutions (DPP) to consider a request to prosecute him in Britain where they believe he would receive a much shorter sentence.

But the Crown Prosecution Service (CPS) said a review had concluded it would be wrong for him to face trial at home.

“These were not random experiments in computer hacking, but a deliberate effort to breach U.S. defence systems at a critical time which caused well documented damage,” said Alison Saunders, head of the CPS Organised Crime Division.

“They may have been conducted from Mr McKinnon’s home computer -- and in that sense there is a UK link -- but the target and the damage were trans-atlantic.”

MILITARY NETWORKS

McKinnon is accused of causing the entire U.S. Army’s Military District of Washington network of more than 2,000 computers to be shut down for 24 hours.

He has told Reuters he was just a computer nerd who wanted to find out whether aliens really existed and became obsessed with trawling large military networks for proof.

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

The CPS decision does not spell the end of McKinnon’s legal battle. Last month, London’s High Court ruled McKinnon, who has been diagnosed with Asperger’s Syndrome, could seek a judicial review of the government’s decision to extradite him.

His lawyers argue his health would suffer and he would be at real risk of suicide if he was handed over to U.S. authorities.

“Mr McKinnon will still remain in the United Kingdom as the judicial review proceedings against the (U.K.) Secretary of State remain outstanding and we are hopeful that those proceedings will be successful,” his lawyer Kaim Todner said in a statement.