Cross-Channel ferry passengers face major Easter travel disruption due to a strike by French port control officers.
The action, starting at 7am UK time on Thursday, is likely to lead to long delays to sailings to most French ports on Thursday and on Good Friday, P&O Ferries warned.
Le Havre is expected to be strike-free, but Calais, Dunkirk, Dieppe and Cherbourg are all likely to be affected by the strike, which will end at 7am on Good Friday.
P&O Ferries said: "We advise that following failed pay talks yesterday (Tuesday) in Paris, French port control officers will be holding a national day of industrial action which is likely to affect the majority of ports in France.
"It is as yet unclear as to the kind of action that will be held which may vary from a 'go slow' to a temporary or total suspension of services for a 24-hour period."
The company went on: "Bar a last-minute breakthrough in the negotiations we must assume that there could be disruption to the cross-Channel services between Dover and Calais and the other French Channel ports.
"We therefore advise our customers of the potential for long delays between Thursday morning and Saturday morning due to the likely knock-on effect of the disruption and queuing."
P&O said tourist passengers were being given the option of either sailing before the industrial action commences or delaying their departures until Saturday when there will be spare capacity.
The company continued: "Anyone who chooses to turn up and wait during the period of the dispute is advised to expect long delays and to prepare accordingly by bringing food and drink.
"Freight customers are being advised to try and arrive at the respective port of embarkation as early as possible in order to remain ahead of any potential delays. The company is in close contact with the French authorities and the British Government in a bid to exert political pressure to resolve the dispute."