Armed troops landed on a cross-Channel ferry as part of a security drill - just days after fears over a terror attack on a ship were raised.

The introduction of sea marshals on vessels has come a step closer after the armed personnel took part in an on-board security exercise.

Fears have arisen over the threat of armed militants launching an attack on a ferry and executing passengers or detonating a suicide bomb.

Brittany Ferries carried out the exercise on the Mont St Michel ferry, travelling from Portsmouth to Caen, as part of moves to make the highly trained military personnel part of the security network.

Announcements in English and French alerted up to 800 passengers that they were in the middle of a security exercise.

Half-way through the crossing, at about 5.30pm, around three hours before the ship's scheduled arrival in France, three armed French sea marshals wearing full military kit landed on the ship by helicopter.

The Irish Army Ranger Wing taking part in a similar security exercise in the Irish Sea (
Image:
Getty)
There are fears terrorists will strike at a civilian ship. File picture of a similar exercise in the Irish Sea (
Image:
YouTube/DFMagazine)
Army Ranger Wing soldiers in an Irish military drill (
Image:
YouTube/DFMagazine)

The marshals, who are effectively part of the French military, patrolled the ship and carried out other security measures.

The exercise was part of a potential ramping up of security in light of recent terror attacks in France and elsewhere in Europe, amid fears there may be more to come.

With the nation on its highest level of security alert, French authorities are keen to be seen to be doing something positive, particularly on French-flagged vessels, to tackle potential security threats.

Public demands in France for increased security have accelerated rapidly in the last few weeks.

A Brittany Ferries spokesman said: "Access to outside decks was not allowed at the time of the helicopter's arrival. The security crew travelled with passengers to France, where they left the ship on foot.

Irish soldiers board a ferry (
Image:
YouTube/DFMagazine)

"Security exercises like today's on board Mont St Michel give Brittany Ferries an opportunity to practice its incident response in partnership with other agencies.

"Safety and security are our highest priority and we operate according to robust procedures set out in the ISPS (International Shipping and Port Security) Code, an international framework endorsed by the International Maritime Organisation (IMO).

An Irish soldier scales the side of a ship (
Image:
YouTube/DFMagazine)

"We very much appreciate passengers' support during this exercise and thank them for travelling with Brittany Ferries."

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 ISIS atrocity.

Army Ranger Wing Exercise in the Irish Sea (
Image:
YouTube/DFMagazine)
An Irish Army Ranger Wing boat during the anti-terror drill (
Image:
YouTube/DFMagazine)

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.”