The UK government has issued an alert to airlines around the world, urging them not to allow former US intelligence contractor Edward Snowden to board flights to the United Kingdom.

The alert, dated Monday on a Home Office letterhead, said carriers should deny Mr 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 yesterday at a Thai airport. A British diplomat confirmed that the document was genuine and was sent out to airlines around the world.

Mr Snowden revealed top-secret documents about US National Security Agency surveillance programmes and is believed to be in Hong Kong. No warrants for his arrest have been issued.

The official said such alerts are issued to carriers that fly into the UK and any carrier that brings Mr Snowden will be liable to be fined £2,000 (€2,356). He said Mr Snowden would likely have been deemed by the Home Office to be detrimental to the “public good”.

Mr Snowden, 29, revealed himself as the source of top-secret documents about US National Security Agency surveillance programmes that were reported earlier by The Guardian and Washington Post newspapers. He is believed to be in Hong Kong.

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.

The document had a photograph of Mr 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 UK government wants to deny entry to people who don’t normally need visas to enter the country.