Heavy traffic is set to hit main routes on what's dubbed as Black Saturday

France has drafted in heavily armed soldiers and police to its ports in fear of ISIS jihadis boarding UK-bound ferries.

The specialist forces have been called upon following intelligence warnings that ferries heading to Britain could be at risk.

Motorists heading to France this weekend have been warned to brace themselves for extreme congestion once they cross the Channel.

French highway authorities are warning that heavy traffic is set to hit main routes throughout the country on so-called Black Saturday, when there is a crossover between holidaymakers heading home and those just beginning their breaks.

The Royal Marines boarded the passenger ferry as it crossed between Ardrossan to the Isle of Arran last year

Excited passengers took photographs of the black-clad commandos as they scaled the side of the vessel

Motorists heading to France this weekend have been warned to brace themselves for extreme congestion once they cross the channel

The Kent port is expected to be busy again over the coming days as travellers arrive to cross the Channel

The heightened risk of a terrorist attack across the Channel comes after the Bastille Day lorry attack in Nice, which killed 84 people and the execution of a priest in a Normandy church.

The French president Francois Hollande said he would increase armed security at transport hubs and crowded areas, but the move at Calais is said to have shocked British authorities.

A senior UK security source told the Daily Mirror: '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.'

Britain's Special Boat Service have been training to deal with ship hijackings.

Last November, the Royal Navy released pictures of a group of Royal Marine Commandos boarding a passenger ferry as part of an hijack rescue training mission.

The elite sailors from 43 Commando sped towards the MV Caledonian Isles ferry as it sailed between Ardrossan to Brodick on the Isle of Arran.

Excited passengers took photographs of the black-clad commandos as they scaled the side of the vessel

Marines from 43 Commando based at HMNB Clyde were involved in the dramatic training mission

The marines, based in HMNB Clyde perform several of the training missions each year.

According to the ferry company Caedonian MacBrayne: 'These exercises usually take place a couple of times a year and the MV Caledonian Isles presents a perfect platform for the training due to her high freeboard - the distance from the waterline to the upper deck level - which allows the commandos to simulate boarding a large cargo vessel, for example, in the case of anti-piracy operations in theatre, when they could be required to climb to a significant height.'

The company said that passengers on board love the drama of the boarding as they are warned in advance about the mission.