Migrants have been found hiding on a freight train in an attempt to reach Britain, heightening concerns that new entry points are being targeted.
The train was stopped in the Channel Tunnel and searched near the opening on the French side after an individual was spotted.
Police found "a number" of migrants on board the train, which was bound for the UK.
It was said to have come from the SNCF Reseau track in Frethun, which is just outside Calais. SNCF Reseau operates the French railway system.
The incident underlines concerns that migrant incursions could shift to different locations after security was stepped up at hotspots such as the main Eurotunnel terminal at Coquelles.
A spokeswoman for Eurotunnel said it had requested that security be increased at the SNCF Reseau yard in Frethun "in line with the security at its own site".
She added: "This incident shows that now that security at the Eurotunnel site has been increased in line with operating volumes, migrants are seeking new routes to cross to the UK."
A migrant was spotted on board the train at around 11am on Friday morning.
The Eurotunnel spokeswoman added: "The alert was raised and the train stopped and searched before it could travel to the UK through the Channel Tunnel.
"The search was conducted by the Police aux Frontiers and security staff who found a number of migrants on board.
"These were returned to France where they will be processed by the PAF and the train was set back to the Frethun Yard."
Delays continued on freight and passenger services on both sides of the Channel. Passengers in the UK were warned they face a waiting time of around 30 minutes before check-in and four hours on the terminal.
Freight drivers on both sides were warned the journey from check-in to arrival in France or the UK would take around two-and-a-half hours. Disruption was also reported on Eurostar services as a result.
Eurotunnel said full services are being restored and there is enough capacity to carry all traffic forecast to travel today.
The spokeswoman added: "We apologise for any inconvenience caused by this incident."
The incident comes a day after Home Secretary Theresa May visited the terminal at Coquelles.
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.
Other ports in northern France, such as Dunkirk, are also likely to come under scrutiny.
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.