The British government has warned airlines around the world to refuse to allow Edward Snowden, who leaked information on top-secret US government surveillance programmes, to fly to the UK.
The US government has begun to take steps to prosecute the National Security Agency (NSA) sub-contractor, who now lives in Hong Kong, for exposing information on wide-scale surveillance programme.
The Associated Press saw a photograph of the document taken on Friday at a Thai airport of a British travel alert, dated on Monday on a Home Office letterhead, that said carriers should deny Snowden boarding because "the individual is highly likely to be refused entry to the UK".
A British diplomat confirmed that the document was genuine and was sent out to airlines around the world. Airlines in Thailand, Malaysia and Singapore confirmed the alert had been issued.
The diplomat, who spoke on condition of anonymity, said such alerts were issued to carriers that flew into Britain and any carrier that brought Snowden would be liable to be fined 2,000 pounds ($3,000).
Snowden, 29, said on Sunday he was the source of top-secret documents about NSA surveillance programmes that were reported earlier by The Guardian and Washington Post newspapers.
The American citizen has yet to be charged with any crime and no warrants have been issued for his arrest. He has vowed to contest any US attempt to extradite him.
He has said he plans to request asylum and that he divulged secrets to British and US newspapers because he believed the US surveillance programmes were illegal and intrusive.
If other countries followed Britain's example and barred his entry, Snowden would have few options for seeking refuge if he were not allowed to stay in his preferred sanctuary of Hong Kong, a semiautonomous Chinese territory.
China has not made any public comment on what it plans to do with Snowden or how long he would be welcome to stay in Hong Kong.
A popular Communist Party-backed newspaper, however, has urged China's leadership to milk Snowden for information rather than expel him, saying his revelations concern China's national interest.
Protesters worldwide have rallied to support Snowden, including in New York and Hong Kong, where about 1,000 protesters were expected to march to the US consulate and then the government to urge the administration of the semi-autonomous territory to not extradite him.
If the US calls for his return, Snowden has the option of applying for asylum or refugee status in Hong Kong, which maintains a Western-style legal system. If Snowden chose to fight it, his extradition to the US could take years to make its way through Hong Kong's courts.
The alert was issued by the Risk and Liaison Overseas Network, part of the UK Border Agency that has staff in several countries identified as major transit points for inadequately documented passengers.
It said that carriers might be liable to costs relating to his detention and removal should they allow him to travel.
FBI Director Robert Mueller told the House Judiciary Committee on Thursday that the disclosures had caused significant harm to the US nation and its safety.
"We are taking all necessary steps to hold the person responsible for these disclosures," he said.
Responding to questions by Bob Goodlatte, a Republican and House Judiciary Committee chairman, Mueller said the Foreign Intelligence Surveillance Court has approved the surveillance programmes and they have been conducted in compliance with US law and with oversight from Congress.
The revelation that the NSA is collecting millions of US phone records along with digital communications stored by nine major Internet companies has touched off a national debate over whether the Obama administration, in its efforts to thwart terrorism, has overstepped proper bounds by using intrusive surveillance methods.