Trespassers on the tracks of the Channel Tunnel and reports of migrants on train roofs caused major disruption for passengers overnight.
Two Eurostar services were eventually forced to turn back, one to London and one to Paris, after those on board had endured long delays at the French port.
Another London-bound train was unable to get back to Paris due to a technical fault and some passengers complained they had been "abandoned" in the early hours of the morning as they waited for alternative transport.
Police had been called to Calais just before 9pm to reports of people on the tracks at the crossing, with hundreds of passengers stuck on carriages for hours while the trespassers were removed.
Some travellers said they had seen migrants on top of other trains, and many complained of their frustrations at the delays and re-routes.
One passenger on the 9055 train from Paris to London told of his anger on learning that he had to wait on alternative transport, having already been re-routed back to the French capital.
Danny Bell tweeted: "We've been abandoned on a platform somewhere in the cold with NO INFORMATION."
Eurostar apologised saying: "It may be possible for another train to be sent to you. We appreciate it is very late now and everyone must be very tired."
While the operator referred only to the people on the tracks as trespassers, a number of passengers said they saw or were told of migrants in the area.
Lucy Francis from London posted online: "the trespassers are on the roof of the train in front of us".
Simon Gentry said: "On a Eurostar apparently 'surrounded by migrants'. Been asked to 'listen out for people walking on the roof' while we wait for the police."
Conor Maples wrote: "Our Eurostar has been stopped outside Calais. Train manager told us there are migrants on the roof. Police are now here. Very very sad".
A spokesman for Eurostar said passengers hoping to travel later this morning should expect a normal service but added that two trains have been cancelled "in order to ensure the service is as robust as possible".
Passengers due to travel on the 7.55am service from London to Paris and those who had booked the 11.13am Paris to London will be transferred to other services at times close to their intended departure, the spokesman said.
Three trains, one from Brussels and two from Paris, were able to continue their journeys to London after the delays this morning.
Anyone affected by the disruption will be compensated, the spokesman said.
He said: "For the passengers affected what we have in place is people waiting for them in stations. We can, if passengers wish, re-book them on the next available train to get to their destination."
He said taxis, hotels and refreshments would be provided.
The operator was inundated with passengers seeking information and venting their frustrations at the delays.
One woman, named Joanne, complained about the temperature: "it's so cold on the train. Give us blankets!!!!"
The crossing at Calais has been plagued with problems all summer amid a growing migrant crisis.
Many migrants are stationed at a camp near the port and on several occasions have broken into the Tunnel and climbed aboard lorries or trains in an attempt to get to the UK.
At its peak the number of attempts to board lorries or trains was around 2,000 a night but that has since fallen.
The crisis in Calais is part of a wider migrant surge into Europe from north Africa and the Middle East.
Eurostar said it was sending an empty train to Calais from London in the "early morning" to pick up stranded passengers of the 9055 service.
Traveller Tom Mayes said he had so far endured a nine-hour delay and described the operator's apology as "not good enough".
He tweeted: "Hungry, tired and fed-up people here. Not good," adding: "children out in the cold here. Disgusting."