All Cross-Channel rail services were suspended on Thursday due to an unexpected strike in France, disrupting tens of thousands of journeys.
The industrial action is affecting Eurostar – which operates passenger services to and from London St Pancras – and Eurotunnel Le Shuttle, which runs vehicle-carrying trains to and from Folkestone.
At least 24 Eurostar trains have been cancelled.
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The M20 coastbound carriageway between Junctions 8 and 9 has also been temporarily closed due to suspended Eurotunnel services, Kent Police said.
The “emergency measure” will allow freights bound for the continent to queue on the empty section of motorway to try to minimise disruption to Kent’s wider road network.
A Eurostar spokesperson said: “Due to a last-minute strike by Eurotunnel staff, no Channel Tunnel crossings have taken place since 12.00pm.
“Unfortunately, Eurostar has had to cancel all trains to and from London until further notice.
“Customers affected will receive direct communication about their journey and can exchange their tickets free of charge or claim a refund.”
The 19.01 and 20.01 services from London St Pancras International are still scheduled to run, according to the Eurostar website.
But passengers booked on those trains were warned that the “situation remains uncertain” and their journeys “may be subject to delays”.
Earlier, several trains were already heading towards the Channel Tunnel when they were forced to return to their starting point.
News agency Reuters obtained a statement from Getlink, which owns the Channel Tunnel and operates Eurotunnel Le Shuttle services, which said: “Today’s call for strike action by representatives of Eurotunnel’s French site staff unions has resulted in the complete interruption of service and the closure of our terminals in France and the UK.”
The company reportedly added that trade unions had rejected an offer of a bonus worth 1,000 euros (£867) per employee, demanding a payment worth three times as much.