Cross-Channel ferry services have been disrupted following a blockade at the Port of Calais.
Ferry movements at the northern French port were suspended due to industrial action, Port of Dover officials said.
Operator DFDS Seaways said its Malo Seaways ship was waiting for permission to enter the Port of Calais.
And there was the possibility that its Calais Seaways vessel may have to be diverted to Dunkirk due to the blockade.
Mayor of Calais Natacha Bouchart said she regretted that the port was being taken "hostage" as travellers were urged to contact their operator before setting off.
A Port of Dover spokesman said: "We will continue to monitor the situation closely in liaison with our ferry partners and the Port of Calais in order to resume normal operations as soon as possible."
P&O Ferries said the Calais port was closed due to action by former MyFerryLink staff. The blockade comes amid a busy Bank Holiday weekend for ferry operators.
Former employees at MyFerryLink have orchestrated wildcat strikes before this summer, causing delays and cancellations for British holidaymakers looking to cross the Channnel.
To add to the difficulties, Eurotunnel said it was only accepting passengers with pre-booked tickets from Calais on Sunday night.