Some ferry services in and out of Calais have been delayed due to protests by workers who have blocked the entrance.
P&O has told its passengers that crew from the recently dissolved MyFerryLink company are demonstrating at the French port.
Ferries are unable to leave or enter the port at the moment, the company added.
Another operator, DFDS Seaways, has said its Malo Seaways ship is currently awaiting permission to enter the port.
It is also possible that another of its vessels, Calais Seaways, may need to be diverted to Dunkirk because of the ongoing blockade.
Disruption for travellers has been compounded by Eurotunnel, which has said it will only accept passengers with pre-booked tickets to leave Calais tonight.
The statement from P&O added: "We have no information how long this will last but negotiations are ongoing to resolve the situation as soon as possible.
"We will endeavour to keep you informed of any change in the situation."
Calais was already busier than usual this weekend owing to the Bank Holiday.
The town's mayor, Natacha Bouchart, has said it is regrettable that the port has been taken "hostage."
Passengers are being advised to contact their operator before beginning to make their journey.
Tara Pahari has been stuck in her car in Calais for several hours:
