A freight train was stopped in the Channel Tunnel amid suspicion migrants were on board.
The train was halted near the entrance to the tunnel on the French side to allow officials to conduct searches.
It was stopped after a migrant was seen on the track, the BBC reported.
A Eurotunnel spokesman in France said: "A suspicion of migrants on a freight train led us to stop and search the train.
"We decided to stop it just at the entrance to the tunnel."
He was unable to say how many people were believed to have been on the train, but he said it was more than one.
The train is now thought to have been moved out of the tunnel, but there has been disruption for both freight and passenger services on both sides of the Channel.
Passengers in the UK face a waiting time of around an hour before check-in and three hours at the terminal.
Freight drivers on both sides were warned the journey from check-in to arrival in France or the UK would take around five hours.
Eurotunnel apologised for the inconvenience. There were also delays of around half an hour on Eurostar services.
The incident comes a day after Home Secretary Theresa May visited the terminal at Coquelles, near Calais.
She inspected new fencing erected in the wake of a surge of migrant incursions before signing a new agreement with the French aimed at easing the crisis.
There is also concern that trafficking gangs could switch their attentions to other European ports after defences were beefed up at Calais.
The Government has opened talks with officials in the Netherlands and Belgium as authorities turn their attention to security at other potential entry points.
The crisis in Calais has receded since its peak earlier in the summer. At one time, around 2,000 attempted incursions were being made every night, but the figure is now thought to have fallen to around 200.