Dubai emirate in particular has become a major tourist destination
British nationals in the United Arab Emirates have been warned there is now a high risk of a terror attack there.
Travel advice from the UK Foreign Office said terrorists might be planning indiscriminate attacks in places frequented by expatriates.
It did not give details about why there was believed to be a more serious threat than the general one previously issued to UK nationals.
Unlike neighbour Saudi Arabia, the UAE has had no major terrorism incidents.
Expatriates make up a majority of the population of the oil-rich state, which consists of seven individual emirates, including the main ones Dubai and Abu Dhabi. It is also a major tourist destination.
"High" is the most serious of four terrorism risk levels which the Foreign Office uses in its warnings, the others being "general", "underlying" and "low".
British officials do not usually comment on the reasons for specific changes in its foreign travel advice, but say conditions in overseas countries are constantly under review.
BBC Security correspondent Frank Gardner quotes British officials as saying the information comes from a number of sensitive sources and they are working hard with the Emirati authorities to address it.
"There is a high threat from terrorism," an official travel advice notice for the country said. "We believe terrorists may be planning to carry out attacks in the UAE."
The web-posted advice says attacks could happen at any time and may target residential compounds, military, oil, transport and aviation interests.
"You should maintain a high level of security awareness, particularly in public places," the advice says.
However, the advice is prefaced by an assurance that the "vast majority of visits" to the UAE are "trouble-free".
At least 100,000 UK citizens are resident in the UAE and more than one million Britons travelled there in 2006, the British embassy says.
Many observers are surprised that Dubai, with its high concentration of westerners and relaxed dress code, has not yet been struck by al-Qaeda-inspired Islamist militants.
But our correspondent says the UAE authorities maintain an extensive network of informants and a number of jihadi militants have been arrested over the years.