Eurotunnel rail services were temporarily suspended in both directions this morning after migrants trying to reach Britain reportedly got on to the platforms in Calais.
Customers who had been waiting on a train at Folkestone were offloaded again as a result of the delays.
As queues built up on both sides of the Channel, the Channel Tunnel operator offered car and van passengers the option to travel by ferry instead.
Lorries on the English side were asked to queue until services were started again, causing tailbacks on the motorway.
The French authorities have reported that the number of migrants living in shanty camps in Calais and hoping to get to England has doubled to 6,000 in recent days.
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Eurotunnel trains have been facing increasingly frequent delays as more migrants take their chances on the tracks, with many each night trying to stow away on the trucks and cars loading on to the freight train. Since June 16 migrants have been killed in the attempt.
At one point during the summer hundreds of migrants tried to rush the boarding area at once.
Eurotunnel said that the overnight rush happened later than usual today, affecting the early-morning trains.
Services were resumed at about 10am, but delays persisted during the morning.