The Foreign Office warned Britons in Thailand to avoid large gatherings
Britons have been warned by the Foreign Office to avoid all but essential travel to the whole of Thailand.
It said the advice "reflects the increasing tension" and fears that violent incidents are occurring across the country, not just in Bangkok.
It follows a series of blasts near a large encampment of anti-government protesters in Bangkok last week, which was already the subject of a warning.
Britons already in Thailand are advised to stay indoors and monitor the media.
And they have been urged to regularly check Foreign Office travel advice.
Britons already in the country have also been advised to consider whether it is essential to travel within Bangkok, the country's capital, and to take alternative routes where possible.
Many rail and coach services to Suvarnabhumi airport pass through the city's centre.
Foreign Office advice issued on Thursday warned Britons against travel to Bangkok, but this has been extended to cover the rest of the country.
The country has seen growing tensions between red-shirted anti-government protesters and security forces, often ending in violent skirmishes.
More than 26 people have been killed and nearly 1,000 wounded in clashes.
A Foreign Office statement said: "British nationals should continue to exercise extreme caution throughout the country and avoid demonstrations and large gatherings.
"This advice reflects the increasing tension in Thailand and our concern that violent incidents of an unpredictable nature are occurring in many parts of Thailand.
"The situation is volatile and we recommend that British nationals in Thailand or planning to travel to the country check the travel advice on the embassy website regularly for details of further developments."
Thailand's prime minister Abhisit Vejjajiva, who has broken off negotiations with the protesters occupying parts of central Bangkok, had said he hoped to resolve the crisis without resorting to force.
However, hundreds of soldiers armed with automatic weapons have been deployed to guard stations and other major city streets.