Gary McKinnon, the man who allegedly broke into the US military and NASA computer systems, began his fight against extradition to the US yesterday.

Edmund Lawson, QC, who is defending the 41-year-old, told a hearing at London’s High Court that, if extradited and convicted, he could face up to 60 years in prison.

What’s more, he told the court that his client could face the prospect of serving his whole sentence in the US, rather than being repatriated to complete part of the prison term in the UK.

Lawson also revealed that the US authorities had offered McKinnon a plea bargain, under which he would receive a shorter sentence if he pleaded guilty and stopped fighting the extradition. This deal was rejected by the defendant.

“If he entered into a plea agreement the prosecution said that they would consent to an extradition hearing in the US and consent to McKinnon’s repatriation in the early course of his sentence,” said Lawson. “That, we contend, was an improper threat that could engage this court’s abuse of process jurisdiction.”

McKinnon’s case dates back to 2001, when it is alleged that he hacked into 97 US government computers, including those of the Pentagon, US Army, Air Force and NASA. US prosecutors accuse him of accessing hundreds of military machines, which had not been secured properly by officials.

The former computer systems administrator admitted breaking into the systems using only a dial-up connection and default passwords, but claimed he was looking for evidence of UFO activity and was only motivated by curiosity.

However, US authorities maintain that he committed the “biggest military computer hack of all time” and caused around $700,000 (£400,000) worth of damage. Extradition proceedings began in 2005.

The case continues.