The Islamic State threat to Britain and Europe will increase because of President Donald Trump's decision to withdraw US forces from Syria, serving and former military and security officials have warned.
The departure will hamper the ability of US and other coalition troops and intelligence officers to keep track of extremist suspects still at large in the region, they said.
It will also damage a crucial counter-terrorism partnership with a Kurdish-dominated militia on the ground. The partnership with the Syrian Democratic Forces was instrumental in the destruction of the IS group's self-proclaimed caliphate.
The continued detention of some 11,000 suspected IS fighters - including about 2,000 foreigners - held in camps in north east Syria is also at risk.
More than 100 detainees are already known to have escaped in the wake of a Turkish offensive against the Kurds that was launched after President Trump said on 6 October he would move his troops out of the way.
A senior European security source said he expected a significant resurgence by Islamic State and al Qaeda, which also has a presence in Syria.
Tory MP Tobias Ellwood, a former defence and foreign office minister, told Sky News: "The threat level in Britain no doubt will have to increase because of this.
"The international community must act swiftly to first recognise… the Kurds need help in processing [IS detainees] to make sure they can't fight, and we have to face the reality that those Daesh [Islamic State] forces are starting to regroup."
The threat level for international terrorism in the UK is already at severe - its second highest level. That means a terrorist attack is highly likely.
The highest level is critical, which means an attack is highly likely in the near future.
The Joint Terrorism Analysis Centre, based at MI5's headquarters in London, is charged with assessing the threat level.
It will doubtless be studying the possible impact of events in Syria over the past two-and-a-half weeks on the wider terrorist threat picture.
Britain and its allies are understood to be adapting their posture to adjust to the rapidly changing reality on the ground in northeast Syria.
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Personnel will be trying to limit the damage they believe the US president's action is having on their counter-terrorism efforts.
General Sir Richard Barrons, a former commander of Joint Forces Command, said any mass breakout from a detention camp in Syria would undermine five years of hard and dangerous work by British, US and other coalition forces to combat the IS threat.
"These people [escapees] will run with the wind and some of them will bring great harm to the UK and other places," he said.
"It matters to us because our own national security is at issue here - if people who are released as a result of this US decision return to the business of terrorism and project that terror in our own country or against our citizens abroad."
General Barrons added: "The threat level is already significant.
"We know that we face an enduring risk from violent, religious extremist terrorism to the UK and to our citizens abroad. What we have done as a result of this American withdrawal is add power and capacity back to ISIS."
Emman El-Badawy, head of research at the Tony Blair Institute for Global Change, said groups like Islamic State and al Qaeda can only be confronted by allies working together, including the United States.
"Unfortunately, this fallout … weakens our coalition against ISIS," she said.
She expected any recalibrated Islamic State to look different to the group that captured swathes of Iraq and Syria in 2014.
But Ms Badawy warned it would be "just as deadly and destructive".
The group, she said, would plant "themselves deeper into weakly governed areas and establishing safe havens for recruitment and training, fuelling instability and no doubt continuing to plot attacks abroad, including [against] Europe and the US".
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