LONDON – The British government has warned airlines around the world not to allow Edward Snowden, who leaked information on top-secret U.S. government surveillance programs, to fly to the United Kingdom.

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 U.K.”

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.

In London, Home Office officials refused Friday to discuss the travel alert.

The diplomat said such alerts are issued to carriers that fly into the U.K., and if any airline brings Snowden into the country, it will be liable to be fined 2,000 British pounds ($3,100). 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 wasn’t authorized to discuss the matter publicly. The U.K. Border Agency, which operates under the Home Office, has wide leeway to deny entry to people trying to enter the United Kingdom. It has been used to keep radical preachers and extreme right-wing politicians out of Britain.

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 publicly charged with any crime and no known warrants have been issued for his arrest.

U.S. Attorney General Eric Holder told reporters in Dublin on Friday that the case was still being investigated, but said he is “confident that the person who is responsible will be held accountable.”

Experts believe Snowden’s travel options are narrow because of the intense publicity generated by the case and the wide circulation of his photo, which is also contained on the carrier alert.

“He’s cornered,” said Magnus Ranstorp, a research director at the Swedish National Defense College. “Even without the U.K. alert, his name is now part of the intelligence matrix and his name would be flagged if he tried to travel anywhere in the world. He can’t get into mainland China, they have a very sophisticated database and sneaking in is not easy, and if he tries to fly he won’t even get out of Hong Kong airport.”