Eurostar trains from Paris to London were hit by cancellations and "severe delays" on Sunday as French customs officers staged work-to-rule industrial action.
The customs officers are demanding higher pay and better working conditions while seeking to demonstrate what might happen if full border controls are put in place once Britain leaves the European Union.
Paris-to-London trains were experiencing "severe delays and lengthy queues for our services," Eurostar said on its website. "We strongly recommend that you do not travel today."
Four trains had been cancelled by lunchtime on Sunday, with another three on Monday and one on Tuesday.
Sunday's work-to-rule was just the latest in a string of strike actions by the French customs officers.
Work-to-rule strikes began in early March, in the Channel ports of Dunkirk and Calais, northern France, leading to long delays for trucks waiting to cross to Britain.
The customs workers want better pay but also more staff to cope with British travellers who will no longer have European passports once the UK leaves the European Union.
Brexit is due to happen on March 29 but looks increasingly likely to be delayed as the British parliament is yet to agree on a divorce plan.
On Wednesday French unions representing the around 17,000 customs workers rejected a government offer of a 14 million euro (USD 15.8 million) payroll boost, saying it was insufficient.
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