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 Friday 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 diplomat said such alerts are issued to carriers that fly into the U.K and any carrier that brings Snowden will be liable to be fined 2,000 British pounds. He said Snowden would likely have been deemed by the Home Office to be detrimental to the "public good."
The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.
Snowden, 29, revealed himself Sunday as the source of top-secret documents about U.S. National Security Agency surveillance programs that were reported earlier by the Guardian and Washington Post newspapers. He is believed to be in Hong Kong.
Snowden, an American citizen, has yet to be charged with any crime and no warrants have been issued for his arrest.
Even without charges, Snowden's world is now shrinking. 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 the U.S. eventually calls for his return, Snowden does have 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 U.S. could take years to make its way through Hong Kong's courts.
The alert was issued Monday by the Risk and Liaison Overseas Network, part of the U.K. 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 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.
"Carrier alerts" are issued when the U.K. government wants to deny entry to people who don't normally need visas to enter the country, or already have visas but something has happened since they were issued, said the diplomat. Sometimes convicted sex offenders are denied entry into the U.K. in this way.
A Bangkok Airways officer said the airline was notified on Thursday about the alert by the Airports of Thailand, Pcl., which operates national airports throughout the country. She said the notice was not intended to be seen by the public.
The officer spoke on the condition of anonymity because she was not authorized to give the information to the media.
National carrier Malaysia Airlines said in an emailed statement to the AP that it had also received the British advisory and issued notices to all its operating locations in the country. Singapore Airlines also received the alert.
Britain previously found itself wrapped up in a secret documents leak scandal when Wikileaks founder Julian Assange was granted political asylum last year at the Ecuadorian Embassy in London. He is facing extradition to Sweden where he is wanted on accusations of sex crimes, and has expressed fears that if returned to Sweden he could also face extradition to the U.S.
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