The Port of Dover said it had suspended services to Calais from 0700 GMT and that loading was expected to resume in the afternoon. Freight traffic on the British side had been contained within the ferry terminal, the Port of Dover added.

"Unfortunately at this time we cannot guarantee our sailing schedule due to industrial action in France," P&O Ferries said on Twitter.

P&O said it hoped its first service would leave Dover at 1400 GMT, while the first ferry out of Calais was expected to depart at 1600 GMT.

Calais-Dover is the shortest sea route between Britain and the European Union - just 23 miles (37 km). Calais, France's busiest road freight port, handles some 2 million trucks per year.

A Reuters photographer said the trucks were being held near Dunkirk and that the queue snaked back for several kilometres.

French workers were striking and joining marches across the country on Thursday, halting trains and cutting electricity production in protest against government plans to raise the retirement age by two years to 64.

(Reporting by Richard Lough; Editing by Frances Kerry)