Alleged computer hacker Lauri Love who lives in Newmarket is beginning a High Court challenge against his extradition to the US.
Authorities in America have been fighting for Mr Love, who has Asperger syndrome, to face trial on charges of cyber-hacking, which lawyers have said could mean a sentence of up to 99 years in prison if he is found guilty.
The Lord Chief Justice Lord Burnett and Mr Justice Ouseley will hear the action over two days in London.
The 32-year-old, who lives with his parents near Newmarket in Suffolk, is alleged to have stolen huge amounts of data from US agencies, including the Federal Reserve, the US Army, the Department of Defence, Nasa and the FBI in a spate of online attacks in 2012 and 2013.
- In September 2016 a district judge at Westminster Magistrates' Court ruled that Mr Love could be extradited.
- In November 2016 Home Secretary Amber Rudd authorised Mr Loves' extradition.
Mr Love's supporters have accused Prime Minister Theresa May of failing to raise the case with Donald Trump in the hope he would intervene and order US authorities to allow him to be tried in the UK.
And a cross-party group of 73 MPs warned recently that there was a "great probability" that Mr Love would end his own life if sent to the US.
Their warnings echoed worries raised in October 2016, when more than 100 MPs wrote to then-president Barack Obama calling on him to block the extradition, stressing their "deep concern" for Mr Love's safety.
His case bears similarities with that of Gary McKinnon, another alleged cyber-hacker with Asperger syndrome, who eventually had his extradition blocked by Mrs May in 2012, when she was home secretary, after a decade-long legal battle.