The British government is reported to have warned airlines around the world not to allow the National Security Agency whistleblower Edward Snowden to fly to the UK.

A travel alert, dated Monday 10 June on a Home Office letterhead, said carriers should not allow Snowden to board because "the individual is highly likely to be refused entry to the UK".

The Associated Press news agency saw a photograph of the document taken on Friday at a Thai airport. A British diplomat confirmed to AP that the document was genuine and was sent to airlines around the world. A Thai airline official also confirmed the alert had been issued.

The official said such alerts were issued to carriers that flew into the UK, adding that any carrier bringing in Snowden would be liable to a £2,000 fine. He said it was likely the Home Office had deemed Snowden's presence in the UK to be detrimental to the "public good".

The Home Office refused to confirm or deny the report, saying only: "We do not comment on individual cases."

Snowden, 29, has revealed himself as the source of top-secret documents about American NSA surveillance programmes that were reported earlier by the Guardian and Washington Post. He is believed to be in Hong Kong. The US citizen has not been charged with any crime and no warrants have been issued for his arrest.

The alert was issued by the Risk and Liaison Overseas Network, part of the UK Border Agency, which 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." Airlines allowing him to travel would "be liable to costs relating to the individual's detention and removal", it said.

According to the diplomat, carrier alerts are issued when the UK government wants to deny entry to people who don't normally need visas to enter the country, or whose visas were issued before something happened.

He said the alerts were sometimes issued to deny convicted sex offenders entry to the UK.

A Bangkok Airways officer said the airline was notified on Thursday about the alert by the Airports of Thailand, which operates national airports throughout the country. She said the notice was not intended to be seen by the public.

British Airways was unable to comment on the alert. A member of staff referred the Guardian to the Home Office, adding: "We comply with government requirements."

Virgin Atlantic, which flies direct to London from Hong Kong, said it would not comment on security issues.

It was not clear if governments in other countries had issued similar documents.

However, Air France KLM, which connects China to Britain via the hubs in Paris and Amsterdam, said it had not received any alerts regarding Snowden.

The WikiLeaks founder Julian Assange criticised the move, describing Snowden as a hero.

"The British government should be offering Mr Snowden asylum, not excluding him from their borders. Edward Snowden is a hero – his revelations disclose something that is important to almost everyone in the world," said Assange, who has been inside the Ecuadorian embassy in London for almost a year. He is wanted in Sweden for questioning about allegations of sexual offences and has expressed fears that if returned to Sweden he could also face extradition to the US over the leaking of secret US logs on the Afghanistan and Iraq wars.

In Hong Kong, where a "Support Snowden" rally organised by human rights and other groups will march past the US consulate on Saturday afternoon, two pan-democratic lawmakers urged the US president to stop legal action against Snowden and consider letting him go.

"I'm actually asking the United States not to even try to charge [Snowden]," Claudia Mo Man-ching of the Civic party told the South China Morning Post on Friday.

Mo and Gary Fan Kwok-wai of the NeoDemocrats have written an open letter to Barack Obama urging him to ensure "national security" claims are not used to justify abuse of state power.

It argues: "While we accept that Mr Snowden may be deemed to have contravened US law, we believe he may have done liberal democracy a service by stimulating serious discussion in many countries of the extent to which surveillance is acceptable in liberal, democratic societies … We urge you to tread very carefully and take account of the views of America's democratic friends around the world."

Mo told the Guardian: "The government should not keep secrets from the people. People should be able to keep some secrets from the government."

Hong Kong's legislative council has said it will discuss the case at a meeting next Wednesday, following a request from lawmaker Ma Fung-kwok, who said the public needed more information on a possible surrender request from the US and Snowden's claims that the US hacked Hong Kong targets.

The Hong Kong chief executive, Leung Chun-ying, has repeatedly refused to comment on the case.