The Foreign Office is urging travellers destined for Bangkok to cancel their plans following violent outbursts in the Thai capital.
It is now advising against all travel to the city in view of the "highly uncertain" security situation. In a statement it said: "We judge that the risk to the safety of British nationals has increased and have amended our travel advice accordingly."
There have been violent incidents in a number of locations across Bangkok. A curfew has been imposed in the city and in 23 provinces.
British nationals already in Bangkok were urged to stay indoors and monitor FCO travel advice and the media.
The FCOs added: "We have also decided to advise against all but essential travel to Chiang Mai due to the worsening security situation there. We continue to advise against all travel to the Preah Vihear temple area and advise against all but essential travel to the far southern provinces of Pattani, Yala, Narathiwat and Songkhla."
The advice against all travel to Bangkok does not apply to travellers transiting the airport en route to other destinations, either internationally or within Thailand.