Julian Assange, the founder of WikiLeaks, said he fears he will die in a US jail if PM David Cameron allows him to be extradited.
He claimed there was a “high chance” of him being killed inside prison in America.
Mr Assange is currently on bail facing extradition proceedings to Sweden on charges of sexual assault.
However there are reports that the US authorities are considering whether they could extradite him to stand trial for espionage over the leak of tens of thousands of diplomatic cables.
Mr Assange said that he believed that it would be “politically impossible” for the British to hand him over to the Americans if there was strong public support for him in the UK.
“It’s all a matter of politics. We can presume there will be an attempt to influence UK political opinion, and to influence the perception of our standing as a moral actor,” he said.
He added: “Legally the UK has the right to not extradite for political crimes. Espionage is the classic case of political crimes. It is at the discretion of the UK government as to whether to apply to that exception.”
He also said that if was extradited to the US, there was a “high chance” of him being killed “Jack Ruby-style” – a reference to the man who killed Lee Harvey Oswald before he could stand trial for the murder of President John F Kennedy.
Jack Ruby died of lung cancer in 1967 three years into his prison sentence for the assassination of Lee Harvey Oswald in 1963. Telegraph