Britain's Home Office have issued a security notice to airlines across Southeast Asia, advising that they should deny Snowden passage on UK-bound flights because "the individual is highly likely to be refused entry to the UK", the Associated Press reported.
The news agency published a photograph of a "carrier alert" on British Home Office letterhead taken at Chiang Mai airport in northern Thailand on Friday, and reported that a British diplomat had confirmed the document was genuine.
"We have received a notice and we have issued it internally systemwide," the diplomat said in a text message.
The notice was marked as a "message from UK border", she said. "Apparently the notice came from UK Border. So don't allow him on flights to UK."
A Malaysia Airlines spokeswoman in Kuala Lumpur confirmed to AFP on Friday that the carrier had received a notice asking it not to allow Snowden to board flights to the UK.
The United States launched a criminal investigation after Snowden, a former CIA technical assistant, blew the lid on the National Security Agency's vast electronic surveillance operation.
On Friday, US Attorney General Eric Holder said he was confident Snowden would be prosecuted for "extremely damaging" leaks.
The 29-year-old, who remains in hiding in Hong Kong, has vowed to fight any bid to extradite him.
Wikileaks founder Julian Assange, who is holed up in the Ecuadoran embassy in London, said Friday he believed the UK wanted to block Snowden from entering Britain because it did not want "another Assange" on its hands.
"The British government refused entry to this country to Edward Snowden preventively. Why? Presumably because it doesn't want to end up with another Assange," the Australian told AFP and other agencies in an interview at the embassy.
"Snowden is a hero, his revelations are important to nearly everybody in the world.
"The British government should be offering Snowden asylum, not excluding him. The rule of law has gone astray in the UK -- this is another example," he said.
Assange has been at the embassy for almost a year in a bid to avoid extradition to Sweden where prosecutors want to question him over allegations of rape. He fears he will then be extradited to the US to face trial for a massive leak of US diplomatic and military documents.
In Hong Kong a Cathay Pacific Airways spokesperson told AFP it would "not discuss communications, if any, received from governmental agencies" due to security reasons.
The city's airport authority said it had not received any instructions about banning Snowden from boarding flights.
Virgin Atlantic's press office in Britain said it would not comment on issues of "national security".