Migrants desperately trying to reach Britain have started targeting Eurostar trains because of increased security measures at the Channel Tunnel, operator Eurotunnel has said.
Passengers stranded in northern France overnight described being "surrounded by migrants" who blocked tracks and climbed on to train carriages in their efforts to enter the UK.
Two trains were forced to turn back, one to London and one to Paris, while a rescue train was sent for passengers left in "disgusting" conditions when their service broke down near Calais.
Eurotunnel said security measures introduced at the Tunnel in recent weeks - including extra fencing and more patrols -meant migrants were trying to break on to nearby tracks used by Eurostar and freight trains at Calais-Frethun station.
Eurotunnel spokesman John Keefe said: "Since Channel Tunnel security increased 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."
Home Secretary Theresa May insisted last month that new security measures were taking effect at Calais when she toured a £7 million ring of steel surrounding the Eurotunnel terminal at Coquelles.
But Ukip MP Douglas Carswell said the Government should have anticipated that migrants would seek other ways to enter Britain after security was increased at the Tunnel.
He said: "We've only got one rail link with mainland Europe and yet it seems to be beyond the capability of our Government to keep it open and secure.
"On the basic idea of safeguarding a rail link, our officials are clueless.
"You would have thought the Government would have anticipated this problem would occur. It's a problem which has been running for several months."
Eurostar suggested the disruption overnight 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 who were disrupted.
French police were called to Calais just before 9pm following reports that migrants were on the tracks and climbing on top of train carriages.
One London-bound train was unable to get back to Paris due to a technical fault so passengers were forced to wait for hours on the train and later at Calais-Frethun.
Exhausted passengers arriving at London's St Pancras International today claimed they were left on the train for hours with no food, in darkness and overbearing heat after the power was cut, then packed into a cold station after the carriages were finally evacuated.
George Golesack, 34, from Bethnal Green, east London, said: "People were trying to climb on the train. They were searching for two hours because they said there were people on the roof.
"One woman had a massive panic attack and another woman who was pregnant had to be taken to hospital."
Bridget Roussel, 52, from Greenwich, south-east London, said: "Just before we got to the Tunnel it was chaos. The lights went off and the air conditioning went off, it was so hot.
"It was just disgusting. I've never experienced anything like that in my life. The worst thing was that there was no communication whatsoever."
Michael Richardson, 45, said: "The worst thing was the train was incredibly hot because the power was cut.
"There was a pregnant woman who was very, very ill and threw up violently over a few of the other passengers. Someone came off covered in red vomit."
Mr Richardson also claimed the train doors had been "manually bolted" shut to prevent migrants boarding carriages and a window had been smashed.
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."
Three other trains caught in the disruption, one from Brussels and two from Paris, continued their journeys to London after delays of up to three hours, Eurostar said.
A spokesman for the company said passengers should expect a normal service today but added that two trains have been cancelled "to ensure the service is as robust as possible".