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.
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."
Mr Hague added: "This is a very serious blow to al Qaida but, like any organisation that has suffered a serious blow, they will want to show in some way that they are still able to operate."
The Foreign Secretary, visiting the Egyptian capital Cairo, said the "removal" of bin Laden was a "very, very positive development".
"It is unequivocally a good thing that he is no longer able to pursue terror, murder and mayhem in the world," he said.
But he stressed that it did not mean an end to the international military mission in Afghanistan, from where al Qaida launched the 9/11 attacks in 2001.
"Just as we should be clear that this is an important and positive development, we should also be clear that the problems we are dealing with have not gone away," he said.
Former home secretary and defence secretary Lord Reid warned that there should not be the "least bit of complacency" following bin Laden's death. He said: "I would caution against premature celebration. Al Qaida have lost a major, major figure but they are not finished. It is precisely at this time that our opponents are at their most dangerous."