Wikileaks founder Julian Assange will appeal to the International Court of Justice if Britain prevents him from going to Ecuador, according to a senior Spanish human rights lawyer.
Baltasar Garzon, who is working on Assange's defence, told Spanish newspaper El Pais that Britain was legally required to allow Assange to leave once he had diplomatic asylum.
"What the United Kingdom must do is apply the diplomatic obligations of the refugee convention and let him leave, giving him safe conduct," he said. "Otherwise, he will go to the International Court of Justice."
Ecuador announced on Thursday that it was offering Assange asylum because it believed he would face persecution and a possible death penalty in the United States, where authorities are furious over the release of thousands of confidential diplomatic cables on his website.
Assange has been holed up in the Ecuadorean embassy in London for two months trying to avoid extradition to Sweden, where he is wanted for questioning over sexual offences claims by two women. He denies the allegations.
His supporters fear that if he went to Sweden, he could from there be extradited to the US.
Mr Garzon said the attempted extradition to Sweden was a ploy to allow the US to exact "political revenge" on Assange.
Mr Garzon, best known for trying to extradite former Chilean leader Augusto Pinochet from London to Madrid on human rights charges in 1998, criticised Britain's threat to arrest Assange at the Ecuadorean embassy, saying this was a threat of "invasion".
The former judge, who was barred from the judiciary in Spain in February for exceeding his authority in probing a corruption case, held a long conversation with Assange, 41, on Wednesday evening, the paper said. "He was very confident that they would give him asylum, as they did," Mr Garzon was quoted as saying. "He seemed very calm and in good spirits. He knows he is in the right."
British Foreign Secretary William Hague told London's Times newspaper that he had not expected Ecuador to make public a letter he had sent in which he warned he had the power to strip the embassy of its diplomatic status in order to allow police to enter. He later told reporters: "There is no threat here to storm an embassy."
Assange's quarters come with an air mattress laid on an office floor, and a window from which he can gaze in the direction of London's distant airports, and the possibility they represent of a flight to Ecuador.
For now, that flight might as well be a million kilometres away, given the 20 or 30 Scotland Yard officers keeping a 24-hour watch outside the embassy. Friends who have visited Assange say he has a computer and a broadband connection, at least one mobile phone, and regular deliveries of takeaway food, carefully inspected by the police.
While the British laws governing consular premises do allow for the de-recognition of an embassy, any such move would also have to comply with international law.
Mr Hague said it was a matter of regret that Ecuador had decided to grant asylum and Britain would not permit Assange safe passage out of the country. He said the case could go on for a considerable time.
WikiLeaks said on Twitter that Assange would give a live statement "in front of the Ecuadorean embassy" tomorrow at 2pm, London time. It is not clear what "in front of the embassy" means. The embassy is an apartment inside a much larger building, and an announcement at the apartment's front door would still be inside the building.
He could be seized if it is deemed he has stepped outside the embassy's diplomatically protected area.
The lawyer for the two Swedish women, Claes Borgstrom, said Ecuador's move was absurd and an abuse of asylum law, which was designed to protect people from persecution and torture if sent back to their country of origin.
"He doesn't risk being handed over to the United States for torture or the death penalty. He should be brought to justice in Sweden," Mr Borgstrom said.
The Union of South American Nations will meet tomorrow to discuss the situation at the embassy.
-Sydney Morning Herald