The British government has warned airlines around the world not to allow Edward Snowden onto flights to Britain, marking the first official measure to target him even though he has yet to be charged with any crime and no warrant has been issued for his arrest.
If other countries follow Britain's example and block his entry, Snowden will have few options for seeking refuge if he is not allowed to stay in Hong Kong.
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 UK".
A British diplomat said the document was genuine and had been sent to airlines around the world. Airlines in Thailand, Malaysia and Singapore also confirmed the alert had been issued.
In Hong Kong, a spokesman for Cathay Pacific said: "For privacy and security reasons, it would be inappropriate for us to discuss communications, if any, received from government agencies. It would be up to the sending agency to share the information it deems appropriate."
A Hong Kong Airport Authority spokesman said it was not aware of the advisory.
The British diplomat said such alerts were issued to carriers that fly into Britain and that any carrier flying Snowden in would be liable for a fine of £2,000 (HK$24,300). They said Snowden would probably have been deemed by the Home Office to be detrimental to the "public good".
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 Snowden and gave his date of birth and passport number. It said: "If this individual attempts to travel to 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 British government wants to deny entry to people who do not need visas to enter the country, or already had visas but something had happened since they were issued, said the diplomat. Sometimes convicted sex offenders are denied entry into the UK in this way.
Additional reporting by Ng Kang-chung