Special forces are on alert for counter-terror strikes at sea amid fears jihadis could sneak into the UK disguised as asylum seekers on ships.
It comes after the SBS stormed oil tanker Nave Andromeda off the south coast on Sunday to tackle seven violent stowaways.
The Mirror can reveal refugees are ghosting on to boats on inland waterways – adding to the risk of terrorists travelling to Britain through people-smuggling routes.
A security source said of the Nave Andromeda raid: “Special forces have been practising for an event like this for many years but lately their training has increased as it is thought to be only a matter of time before there is a more serious taking of a ship, possibly by terrorists.
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“The Nave Andromeda mission was textbook and a huge success but the worry was there was a chance it could have been more sinister than an attempt by illegals.
“From the moment news broke that a ship may have hostile illegals on board it was always going to become an armed counter-terror mission, just in case.
“With limited information coming from the ship at the time the Home Office realised it had to end the stalemate and deploy the SBS to retake the ship.
"It ended well but if they had been armed or carrying explosives it would have been a very high-risk operation.”
The Nave Andromeda stow- aways had apparently boarded in Lagos, Nigeria, where a group with links to Islamic State has grown in recent years.
After a 10-hour stand-off the Home Secretary agreed to hand over to the military and the SBS used five helicopters and rapid attack boats to retake the tanker off the Isle of Wight.
Hampshire Police arrested all seven stowaways “on suspicion of seizing or exercising control of a ship by use of threats or force under the Aviation and Maritime and Security Act 1990”.
And we can reveal four migrants were also taken off a cargo ship near Brighton on Thursday after boarding in Rouen, France, a port 80 miles inland.
The craft, called Alessandra Lehmann, sailed along the Seine river into the open sea and the stowaways were only spotted when it reached the UK.
The group were then taken to a police station.
Thousands of migrants have crossed the Channel in flimsy dinghies this summer but this is thought to be one of the first times on which a group boarded a ship inland to get here.
Asked for a response to the Alessandra Lehmann incident, the Home Office said: “We are fixing our broken asylum system. An asylum system should provide safe haven to those fleeing persecution, oppression or tyranny.
"But ours enables organised criminals to elbow the most vulnerable to the side.
“Which is why the new system will be fairer and firmer.
"It will be compassionate towards those who need our help.
“And it will stop exploitation by those who come here illegally and often make unfounded or meritless claims to remain.”