The UK is also on guard against an Islamic State cell thought to be plotting attacks across Europe.
It is believed 800 Britons have gone to fight for ISIS in Syria and Iraq, with up to 360 now returned and seen as a serious, radicalised threat.
Security services are having to prioritise between jihadis who have had their fill of carnage, fanatics bent on bringing war to Britain’s towns and those who will become “sleepers” – biding their time before being activated for a mission by IS.
Meanwhile, any country taking part in the US-led coalition currently bombing IS in Iraq and Syria, is thought to be high on the terror group’s European hit list.
France, which was hit by the Paris terror attacks in which IS killed 130 in November, and Belgium, which is still reeling from Tuesday’s massacre of at least 31 people, have both taken part in coalition air raids.