The Channel Tunnel was shut today and services through it suspended as migrants caused chaos in Calais by attempting to board UK-bound lorries.
High-speed train company Eurostar said services were being cancelled for the rest of the day and warned passengers not to go to stations after striking ferry workers broke into the tunnel.
Three trains stuck outside the tunnel on the English side had to return to St Pancras in London while other trains on the French side were taken back to Paris and Brussels.
The disruption came as migrants tried to to exploit wildcat strike action by the French ferry workers.
Lorry drivers forced to slow or stop due to queuing traffic on approaches to the northern French port were faced with migrants attempting to clamber on board.
Hundreds of Eurostar passengers were left stranded at St Pancras International in London as they waited to find out if trains would resume.
Martin Nicholls tweeted: "Chaotic scenes in St Pancras from panicked travellers. Eurostar staff very visible and helping alleviate fraught scenes."
Rachel Kimber tweeted: "Getting pretty chockablock at St Pancras this evening - Eurostar issues."
Steven Ramage wrote: "St Pancras chaos as Eurostar cancelled. Trespassers on tracks & burning tyres apparently. No Brussels for me today!"
Some passengers tweeted pictures of singer Chas Hodges, from Chas and Dave, as he entertained the St Pancras crowds on a piano.
The travel problems come amid a worsening migrant situation near the Port of Calais where numbers camped there have swelled to more than 3,000 since April.
Aid workers have reported a "catastrophic" situation, with predictions that some 2,000 more migrants displaced from war-torn countries including Eritrea, Syria and Afghanistan could arrive over the summer.
British haulier Chris Cary spoke of how he has been stuck in Calais for seven hours, and told of the brazen attempts by migrants to try to get on to lorries bound for the UK.
He told Sky News: "When I was down at the train, (migrants) were trying your doors to get in the cab, trying to sneak under the axles on the trailers, trying to cut the locks and seals on the back of the trailers, even trying to climb on the roof of the trailers.
"Any way they were trying to get in to get to England. This morning was the worst I had ever seen it because there was no police presence."
A Port of Dover spokesman tonight said ferry services between Dover and Calais had resumed after the industrial action in France.
But Operation Stack - where freight traffic is held on the coastbound carriageway of the M20 in Kent - would remain in place due to the volume of freight traffic.
The Foreign and Commonwealth Office (FCO) updated its travel advice on France, urging people to keep vehicle doors locked in slow-moving traffic and when unattended in Calais.
Truckers were also being advised not to stop within about 60 miles (97km) of Calais, to stick with other drivers and make sure padlocks are kept on vehicles.
Donald Armour, the Freight Transport Association's international manager, said: "There is a lot of fighting between the migrants who all want to be on the best part of the road to get on to the lorries.
"We haven't had a fatality but it's not good. The situation is definitely worse than it has ever been."
David Cameron's official spokeswoman said the Prime Minister was "being kept updated" on the situation in Calais.
"We are in close touch with the French authorities on the situation there and we are monitoring the impact of the strike on British people," said the spokeswoman.
The deputy mayor of Calais, Philippe Mignonet, reiterated calls from French politicians for the border to be moved from northern France to Britain.
He told the BBC: "Calais is not the destination. As you've heard, (migrants) want to get into trucks, they want to get to England.
"England has got to realise that it is not our responsibility. The English border is in Calais and I'm requesting, such as (Calais mayor) Natacha Bouchart, for the border to be transferred back in Dover and in Folkestone.
"We can't just accept any more, to be blamed for immigrancy. Again, they want to go to England, they are not coming to Calais, they go through Calais to get to England."