P&O Ferries has suspended sailings “for the next few days” and has sacked 800 seafarers as it is “not a viable business” in its current state.
Workers currently on board ships were instructed by unions not to leave.
Coaches carrying agency workers hired to replace them are parked near ships at ports.
There were reports that balaclava-clad security teams were sent on to ferries to remove the sacked staff.
Mark Dickinson, general secretary of maritime union Nautilus International, said: “The news that P&O Ferries is sacking the crew across its entire UK fleet is a betrayal of British workers.
Three P&O Ferries, Spirit of Britain, Pride of Canterbury and Pride of Kent moor up in the cruise terminal at the Port of Dover in Kent as the company has suspended sailings.
Gareth Fuller
“It is nothing short of scandalous given that this Dubai-owned company received millions of pounds of British taxpayers’ money during the pandemic.”
Holidaymakers who have headed to ports and were ready for their journeys today will be met with the unexpected news and could be looking to claim compensation.
P&O Ferries' check-in lanes at the Port of Dover in Kent.
Gareth Fuller
A statement from the ferry operator on Twitter read: "Regretfully, P&O Ferries services are unable to run for the next few hours. Our Port Teams will guide you and travel will be arranged via an alternative operator. We apologise for the inconvenience this will have on your journey plans".
According to ABTA, a trade association for tour operators and travel agents in the UK, consumer's rights for a refund will depend on the length of time your journey is delayed by.
ABTA rules that if a cancellation results in an overnight stay then accommodation should be offered free of charge to the passenger.
A ferry operator which cancels a service should also offer a refund or an alternative option.
ABTA said: "If your ferry operator expects your departure to be cancelled, the operator should inform you as soon as possible. If this is on the day of travel, you should be informed no later than 30 minutes after your scheduled time of departure."
The trade association states that consumers are entitled to compensation of 25 percent of the ticket price if a service is delayed for: at least one hour for a journey of duration four hours, two hours for a journey of duration between four and eight hours, three hours for a journey of duration between eight and 24 hours or six hours for a journey of duration of over 24 hours.
If the delay exceeds double these times, the compensation will be 50 percent of the ticket price.