The killing of Osama bin Laden is expected to cause a security headache for the cash-strapped Irish police force and intelligence services when President Barack Obama visits the Republic later this month.
And in the UK MI5 is monitoring about 2,000 extremists thought to present a potential terrorist threat, while British embassies and military bases have been put on high alert for reprisal attacks.
Al-Qaida will “undoubtedly” recover and strike back in retaliation, experts said. The US has put embassies on alert and warned Americans of al-Qaida reprisal attacks.
Garda and Irish army intelligence officers are concerned that Islamic extremists, currently keeping a low profile in the UK or on mainland Europe, could enter the Republic to launch an attack when President Obama visits in three weeks’ time.
Both forces are keeping a close watch on contacts between sympathisers located in Ireland and activists overseas and are monitoring their movements and all telephone and computer traffic.
A dozen hardline supporters of groups linked to the al-Qaida network pose the biggest headache for the Irish security agencies involved in combating international terror threats on a daily basis. The prime suspects are mainly people who have lived in the Republic for several years, and represent a tiny minority in the 40,000-strong Muslim community.
Elsewhere, John Gearson, reader in terrorism studies and director of the Centre for Defence Studies at King's College London, said embassies and military around the world will remain on high alert “for some time”.
“I think the significance of what has happened cannot really be overstated,” he said.
“There will be concerns that there could be some sort of retaliation, that al-Qaida may well want to demonstrate that they are still strong and still in the game.”
David Cameron urged the British public at home and abroad to be vigilant amid fears that al-Qaida could attempt a show of strength following Osama bin Laden's death.
Mr Cameron last night chaired a meeting of Cobra, the Government's emergency planning committee, to consider the potential threat of al-Qaida retaliation.
Earlier he spoke by telephone with Pakistan's president Asif Ali Zardari and prime minister Yousuf Raza Gilani, and with Afghan president Hamid Karzai.
“The Prime Minister made clear in the conversations that Britain would continue to work extremely closely with both Afghanistan and Pakistan to tackle the terrorist threat from al-Qaida and from the Taliban,” a No 10 spokesman said.
The Prime Minister, who was woken early yesterday to receive the news from Mr Obama, said: “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.”
Foreign Secretary William Hague said he had instructed embassies to review their security: “There may be parts of al-Qaida that will try to show that they are in business in the coming weeks.”
Defence Secretary Liam Fox, 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-Qaida or its sympathisers”.
Two former Labour Home Secretaries also cautioned against complacency, warning that Islamist extremists could attempt to execute fresh terror attacks in revenge.
“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,” 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.”
Frank Faulkner, a terrorism specialist at the University of Derby, said revenge attacks in the aftermath of bin Laden's death were on the horizon, adding: “It's just a case of when and where.”
“Every security operation in the world will be on the highest state of alert in readiness for any kind of attack by al-Qaida,” he said.
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 particularly alert to the risk of bin Laden's death being exploited by extremists to recruit British youngsters to the jihadist cause.
Radical UK-based Islamic cleric Anjem Choudary said: “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.”