Special forces are on alert for 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.
It is believed that 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.
“The Nave Andromeda mission was textbook 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.
“It ended well, but if they had been armed or carrying explosives it would have been a very high-risk operation.”
The Nave Andromeda stowaways 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”.
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 into the open sea and the stowaways were only seen when it reached the UK.
The Home Office said the group were taken to a local police station.
Thousands of migrants crossed the Channel this summer, but this is thought to be one of the first times on which a group got on a ship inland to get here.
Asked for a response to the incident, the Home Office said: “We are fixing our broken asylum system. The new system will be fairer and firmer.”
