Eurostar passengers have faced major disruption after migrants in France blocked tracks and climbed on to train carriages in a desperate effort to reach Britain.
Two services were forced to turn back, one to London and one to Paris, while a "rescue train" was sent for passengers stranded for about 12 hours near the Calais port.
Three more trains bound for London faced delays of up to three hours while a further two services were cancelled this morning, Eurostar said.
Commuters stranded in France spoke of being "surrounded by migrants", including some who had climbed on top of trains, and branded the situation a "fiasco".
Eurotunnel said extra security measures at the Channel Tunnel meant migrants were now trying to get on to tracks used by Eurostar and freight trains at Calais-Frethun station.
Eurotunnel spokesman John Keefe said: "Since Channel Tunnel increased security over the past few months, we've seen the migrant problem displace to other areas as they try to get on to trains before they reach the tunnel.
"They're stopping trains by blocking the tracks, then trying to get through the tunnel, but we won't accept trains with migrants on."
Eurostar suggested it was the first time its passenger trains had been targeted by migrants during the recent crisis.
Asked whether its service had been disrupted previously by migrants trying to board trains, a Eurostar spokeswoman said: "We've been running a normal service all summer."
The company said the six trains affected overnight could carry up to 4,500 passengers but refused to reveal the exact number of commuters who faced disruption.
French police were called to Calais just before 9pm to reports of people on the tracks as passengers reported that migrants were on top of trains.
Simon Gentry wrote on Twitter: "On a Eurostar apparently 'surrounded by migrants'. Been asked to 'listen out for people walking on the roof' while we wait for the police."
Lucy Francis from London tweeted: "The trespassers are on the roof of the train in front of us".
Conor Maples added "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 London-bound train unable to get back to Paris due to a technical fault was forced to wait at Calais-Frethun station after breaking down at around 10pm local time, Eurostar said.
Eurostar tweeted shortly before 8.50am to say the rescue train had arrived and passengers were waiting for security procedures to be completed.
After the rescue time's arrival time was pushed back to 8.30am, Mr Gentry, who was on board the train, added: "This is becoming a fiasco."
A Eurostar spokesman said passengers should expect a normal service today 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.
Anyone affected by the disruption will be compensated, the spokesman said.
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 about 2,000 a night but that has since fallen.