David Cameron urged the British public at home and abroad to be vigilant amid fears that al-Qa'ida could attempt a show of strength following Osama bin Laden's death.
MI5 is monitoring about 2,000 extremists in the UK thought to present a potential terrorist threat, while British embassies and military bases have been put on heightened guard for reprisal attacks.
The Prime Minister, who was woken early yesterday to receive the news from President Barack Obama, described it as a massive step forward which would be welcomed across the country. But he added: "It does not mark the end of the threat we face from extremist terrorism. Indeed, we will have to be particularly vigilant in the weeks ahead."
William Hague, the Foreign Secretary, said he had instructed embassies to review their security: "There may be parts of al-Qa'ida that will try to show that they are in business in the coming weeks."
Liam Fox, the Defence Secretary, said that he had ordered UK defence facilities at home and abroad to maintain a high level of vigilance "in view of the possibility of violent attacks from al-Qa'ida or its sympathisers".
Two former Labour home secretaries also cautioned against complacency, warning that extremists could attempt fresh terror attacks in revenge.
"Al-Qa'ida 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," Lord Reid said, while David Blunkett warned: "I think we are at more risk temporarily today than we were yesterday. We need to be extremely vigilant."
Britain's security level was raised to severe – meaning that an attack is considered highly likely – just over a year ago. The only higher level, critical, is declared when the security services have specific intelligence that an attack is being prepared.
In recent years, the number of British terror plots linked to Somalia and Yemen has increased, while the proportion of those connected with rural Pakistan has fallen.
MI5 officers will now be alert to the risk of Bin Laden's death being used by extremists to recruit British youngsters to the jihadist cause.
The threat was spelt out yesterday by the radical preacher Anjem Choudary, who was the leader of Islam4UK until it was proscribed.
"Rather than dampening the spirits of those who are today engaged in jihad physically around the world... his death will merely act as an incentive to prove to the world that the death of anyone will not affect them," he said in a statement.
Professor Paul Wilkinson, of the Centre for the Study of Terrorism and Political Violence at the University of St Andrews, warned that a revenge attack by al-Qa'ida was highly likely.
"I think President Obama was right to caution there was likely to be an upsurge in terrorism as a result of the killing of Bin Laden," he said. "Certainly there shouldn't be any hasty winding-down of the specialist services that counter terrorism in the international community."
Lord Carlile, the independent reviewer of anti-terror legislation, said al-Qa'ida sympathisers would be "very active" in seeking revenge. "We have to be particularly vigilant, as the Prime Minister said, over the coming weeks and months. I think there may be a backlash from al-Qa'ida," he said.
Mr Cameron last night chaired a meeting of Cobra, the government's emergency planning committee, to consider the potential threat of al-Qa'ida retaliation.