A travel alert, dated Monday on a Home Office letterhead, said carriers should deny Snowden boarding because "the individual is highly likely to be refused entry to the UK."
The Associated Press saw a photograph of the document taken today at a Thai airport. A British diplomat confirmed that the document was genuine and was sent out to airlines around the world. Airlines in Thailand, Malaysia and Singapore also confirmed the alert had been issued.
The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.
Snowden, 29, revealed himself on Sunday as the source of top-secret documents about US National Security Agency surveillance programs that were reported earlier by the Guardian and Washington Post newspapers. He is believed to be in Hong Kong.
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If other countries follow Britain's example and bar 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.
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 on Monday 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.
The document titled "RALON Carrier Alert 15/13" had a photograph of Snowden and gave his date of birth and passport number. It said: "If this individual attempts to travel to the UK: Carriers should deny boarding." It warned that carriers may "be liable to costs relating to the individual's detention and removal" should they allow him to travel.