P&O has told its passengers not to use its services over the next few days unless it is essential to do so.
It has warned there will be “significant disruption” to its services as it makes changes to its staffing.
The company has reportedly sacked 800 staff and is replacing them with agency staff.
It has instructed all its vessels to stay in port and discharge passengers.
But the RMT union has advised all its members to stay on board and at least one vessel in Hull is reported to be staging a sit-in.
The company says anyone travelling on its Dover/Calais route should arrive at the port as booked and make their way to the DFDS check-in booths where alternative arrangements will be made.
Passengers on all other routes are encouraged to check https://twitter.com/POferriesupdate for the latest advice.
However it adds that if your journey is not essential you should not travel.
Along with Dover/Calais, P&O Ferries also sails between Hull and Rotterdam, Cairnryan and Larne and Liverpool and Dublin.
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