Heavily armed troops and police have been drafted in to French ports to prevent an Islamic State terror attack on a UK-bound ferry.
Dozens of reinforcements have been moved to Calais after intelligence warnings that ferries sailing to Britain are at heightened risk of an IS atrocity.
EU and British spy services are desperate to predict where terrorists might strike next, following outrages including the Nice lorry attack which killed 84 and the execution of a priest at a church altar.
A senior UK security source said: “The presence of the police and soldiers near Calais is very alarming – there’s nothing routine about it.
“Apparently they are at a high state of alert because there is a fear of jihadists getting on to a ferry headed for Britain and executing people on the ship.
“It is also possible that ISIS may have been told security has in the past been reasonably lax so they believe they can get terrorists into Britain via a ferry.”
But security staff on both sides of the Channel are heavily vetted.
And British special forces commandos from the Special Boat Service are trained to board a hijacked ship using high-speed assault boats and helicopters.
A counter-terror source said: “What we’ve learned from the attacks across Europe is that prevention is paramount.
“Once an attack is underway, the terrorists have already got what they wanted.
“Thus far MI5 and the police counter-terror command have managed to prevent scores and scores of attacks, ranging from relatively low-key lone wolf killings to serious spectaculars involving planes.
“Everything that can be done is being done to speed up the process in which the military and police can bring an incident to an end, but it can never be quick enough as people will be getting killed.
“An attack at sea on a ferry is something twisted terrorists want to do because they know the rescue would be slower than on dry land.”
Tonight P&O Ferries – who operate from Calais to Dover – declined to comment on the latest scare for tens of thousands of British travellers.
The ferry alert comes just days after an ISIS attacker slit the throat of French priest Father Jacques Hamel, 86, in a Normandy church.
That horror came soon after the July 14 Bastille Day atrocity in Nice in which a lorry was driven into a crowd celebrating one of France’s main holidays, slaughtering 84.
French counter terror officers and the government have come under attack for failing to stop the attacks, despite the state of emergency.
President Francois Hollande has said France will form a National Guard to prevent attacks and flood transport hubs and crowded areas with armed security.