Two P&O ferries had to sail back to Calais with passengers on board

French fishermen apparently blocked the ports in protest at colleagues being arrested for fishing in banned areas.

Two P&O ferries due to sail back to Dover on Thursday afternoon were stuck in the port and two more had to return to Dover after setting off for Calais.

The blockade came to an end in the evening and the backlog began to clear.

No warning

Long queues of traffic built up on the M20 in Kent and near the French ports after the blockade began at about 1200 GMT.

The fishermen ended their protest at 1815 GMT, with P&O saying it would take two hours to clear the backlog of passengers and traffic.

No warning was given in advance of the protest.

Ferries from SeaFrance and Norfolkline were also affected by the blockade.

Part of the M20 was closed due to a build-up of lorries

At about 1600 GMT, Kent Police launched Operation Stack, which sees part of the M20 closed due to a build-up of lorries unable to get into Dover.

A police spokeswoman said: "The M20 is closed coastbound between junction 11, the Hythe turn-off, and the Cheriton turn-off at Folkestone.

"All coastbound traffic has to leave the motorway at junction 11, lorries are allowed back down onto the motorway where they park up until they can get into the Port of Dover.

"All other traffic is diverted onto the A20."

Later, police said Operation Stack should be lifted by 2200 GMT.

A P&O Ferries spokesman said: "We had been quite busy today and had got a lot of day-trippers across to France.

'Not that bad'

"Now these passengers will have difficulty in getting back."

He said one ship carrying 1,215 people, had to sail back to Dover without being able to dock at Calais.

After the blockade was lifted he said: "We have seven ships which can carry about 600 cars or 120 trucks, so we expect to clear the backlog in about two hours or so, as each sailing is 45 minutes.

"All the ships are in position and ready to load, the backlog is not that bad, it's not even outside the dock gates."

The blockade also affected Norfolkline ferries on the Dover to Dunkirk route.

Geoff Dossiter, of the Freight Transport Association said: "Previous experience suggests that the closure of Calais for a day imposes a cost of around �1m on the transport industry."