Rail traffic under the English Channel was suddenly interrupted on Thursday after an unexpected walkout by railway workers, stranding passengers and causing travel chaos just days before Christmas.

Eurostar, the high-speed passenger train line that runs under the English Channel, said in a statement that 30 trains scheduled to travel out of Paris, London and Brussels had been canceled because of the “last minute” strike at Eurotunnel, the company that operates the rail link between France and Britain.

Four trains were even interrupted mid-journey and had to return to their starting points.

“Eurostar is closely monitoring the situation as it evolves and is being kept informed of developments by Eurotunnel,” the statement said. “Although the situation is out of Eurostar’s control, all our teams are working to minimize the impact on our customers and provide as much support as possible. Extra staff are also in the stations to assist travelers.”

“We’d urge all passengers to postpone their travel due to the Channel Tunnel closure and the continuing uncertainty,” Eurostar said on social media

LeShuttle, the train service that carries cars and trucks through the tunnel, was also suspended, according to a statement posted to its social media accounts, which blamed “French industrial action.”

Six unions representing French railway workers at Eurotunnel said that the purpose of the walkout was a redistribution of the company’s recent revenue that was more favorable to workers.

“For several months now, all the trade unions have been warning senior management of the terrible deterioration in the social climate,” the unions said in a joint statement on Thursday. “This strong turnout comes as no surprise.”

The unions said a meeting with management was underway.

Clément Beaune, France’s transport minister, said that blocking the Channel Tunnel was “unacceptable.”

“A solution must be found immediately,” he wrote on social media. “I am calling upon on everyone to take their responsibilities, to ensure that traffic and holiday departures can continue under good conditions.”

The Eurostar whisks passengers between Paris and London in about two hours and 15 minutes. It also runs between London, Brussels and Amsterdam, though the company said last month that its direct train service from Amsterdam to London would be suspended for six months next year.

Eurostar said in a statement that people traveling on Friday would be updated later on Thursday about potential changes in service and that customers affected would receive information about how they could exchange their tickets without charge or claim a refund.