British embassies around the world have been put on heightened security alert amid concerns of possible reprisal attacks by al Qaida following the killing of Osama bin Laden.
UK Foreign Secretary William Hague said that elements of al Qaida were still "in business" and they would need to be vigilant for "some time to come".
"This is not the end of being vigilant against al Qaida and associated groups," he told the BBC Radio 4 Today programme.
"There may be parts of al Qaida that will try to show that they are in business in the coming weeks, as indeed some of them are.
"So I have already this morning asked our embassies to review their security to make sure that vigilance is heightened and I think that will have to be our posture for some time to come."
Earlier, the US State Department also put American embassies on alert and warned of the heightened possibility for anti-American violence after bin Laden's killing by US troops in Pakistan.
In a worldwide travel alert released shortly after President Barack Obama announced bin Laden's death in a US military operation, the department said there was an "enhanced potential for anti-American violence given recent counter-terrorism activity in Pakistan".
"Given the uncertainty and volatility of the current situation, US citizens in areas where recent events could cause anti-American violence are strongly urged to limit their travel outside of their homes and hotels and avoid mass gatherings and demonstrations," it said.
The alert said US embassy operations would continue "to the extent possible under the constraints of any evolving security situation".
It noted that embassies and consulates may temporarily close or suspend public services, depending on conditions.