Eurostar services are facing "severe disruptions" after two people were struck by trains.
The Foreign Office said: "Eurostar services to and from France will be severely disrupted today (June 25) with some services cancelled.
"Check your operator's advice before travelling, including where you are taking connecting trains."
Eurostar confirmed two separate fatalities between Lille and Paris.
Eurostar has apologised to all customers affected
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Trains were significantly delayed or cancelled in both directions between London St Pancras International and Paris' Gare Du Nord yesterday evening.
So far, at least three trains have been cancelled today, with passengers being left stranded in London and Paris.
Eurostar wrote on X: "Due to a person struck by a train near Lille Europe, we are expecting delays and cancellations to our services this evening.
"We advise changing your journey to a different travel date. Our apologies to customers affected."
The operator is experiencing further delays following "a cable theft near Lille Europe".
Eurostar said: "Our trains are likely to be subject to severe delays and last-minute cancellations.
"We advise you to cancel or postpone your trip. We're very sorry for the impact on your journeys today."
The delays follow a dozen Eurostar trains from London to Paris being delayed or cancelled a week ago, after a person was hit on tracks in France.