The UK government has warned airlines not to allow an ex-CIA worker who leaked secret US surveillance details to fly to the UK.
The Associated Press news agency reported seeing a document at a Thai airport telling carriers to stop Edward Snowden, 29, boarding any flights.
The alert said he "is highly likely" to be refused entry to the UK, AP said.
Singapore Airlines told the BBC it was among airlines to receive the alert. The Home Office would not comment.
AP said the alert was issued on Monday by the Home Office's risk and liaison overseas network.
The agency said the document had a photograph of Mr Snowden, gave his date of birth and passport number and carried the words: "If this individual attempts to travel to the UK: Carriers should deny boarding."
It went on to warn airlines they may "be liable to costs relating to the individual's detention and removal" should they allow him to travel.
According to the Home Office website, a charge for such a situation would be £2,000 ($3,130).
AP said Bangkok Airways and Malaysia Airlines had also confirmed they had received the notice, which was not supposed to be seen by the public.
BBC security correspondent Gordon Corera said the Home Office does have the power to block people's entry to the UK in certain circumstances, such as if it believes it is in the public interest to do so.
Our correspondent said the powers had been used in the past, including to deny entry to extremist preachers and extremist European politicians.
Mr Snowden was last seen in Hong Kong, where he travelled prior to publication of the Guardian newspaper's stories revealing the extent of the National Security Agency's (NSA) programme to take data from US internet and telephone firms.
There is no suggestion that he has any intention of trying to travel to the UK.
Mr Snowden's actions have divided opinion in the US, with some calling him a hero and others calling for him to be tried for treason.