Rail services through the Channel Tunnel have been disrupted because of suspected migrants on the track and even train roofs.
Some passengers on Eurostar were told people had managed to get on top of their train and were asked to "listen out for people walking on the roof".
Reporting the initial disruption shortly before 9pm, Eurostar tweeted: "Trains are waiting to enter the tunnel as trespassers are on the tracks at Calais, the authorities are on site to help."
Following long delays at the French port, two services were forced to turn back, one to London and one to Paris.
Another train heading to London was unable to return to Paris because of a technical fault.
The latest incursion follows weeks of disruption either side of the Channel caused by refugees seeking to reach the UK, prompting authorities to step up security at the Eurotunnel terminal at Coquelles, near Calais.
At one point, around 2,000 attempted incursions were being made every night, but that figure has since fallen.
With thousands more taking increasingly desperate steps to reach western Europe via Hungary, Britain has been told it must do more to ease the "huge humanitarian catastrophe".
A spokesman for German Chancellor Angela Merkel's CDU/CSU alliance warned that Britain's instance on being "out of the club" in sharing the Europe-wide burden could threaten the PDavid Cameron's plans to renegotiate the country's relationship with Europe.
Stephan Mayer told the London Times: "I have always had sympathy and understanding for the British role in the EU and the demands for renegotiation.
"But we are now in such a huge humanitarian catastrophe, I do not have any sympathy or understanding for one-country-orientated positions."
Germany has said it expects to accept 800,000 asylum seekers this year. According to the Home Office, Britain received just 25,771 asylum applications in the year ending June 2015.
Eurostar says a normal service should resume on Wednesday morning but added two trains had been cancelled "in order to ensure the service is as robust as possible".
Late on Tuesday Eurostar passenger Dan Griffey told Sky News: "We were stopped at 9.15pm just outside Calais after leaving Paris and were told we had some trespassers on the roof and around the vehicle, and we have been here ever since.
"The train lost power and was pushed back into the Calais station now which we have just been allowed to disembark.
"Quite some delay - five hours or so without light or water or air conditioning.
"The police inspected the train with a helicopter."
In a Twitter post, Lucy Francis from London said: "the trespassers are on the roof of the train in front of us".
Simon Gentry wrote: "On a Eurostar apparently 'surrounded by migrants'.
"Been asked to 'listen out for people walking on the roof' while we wait for the police."
And Conor Maples said: "Our Eurostar has been stopped outside Calais. Train manager told us there are migrants on the roof. Police are now here. Very very sad".
