P&O Ferries suspend sailings between Liverpool and Dublin ahead of major announcement
P&O Ferries has paused its services "in preparation for a company announcement" later on Thursday.
The ferry operator said it was not going into liquidation but all ferries had been instructed to stay in port.
Union RMT urged the firm to protect jobs amid speculation that hundreds of crewmembers could "be sacked and replaced with foreign labour".
As a result some sailings scheduled for today have been cancelled, with passengers told to use other companies.
P&O services scheduled today include 14 between Dover and Calais and three between Liverpool and Dublin.
The union said it has instructed members to stay on board their vessels once they have docked or risk being "locked out" of their jobs.
"We are digging in for the long-haul. We are determined to fight," RMT spokesperson Geoff Martin said.
A P&O Ferries spokesperson said: "P&O Ferries is not going into liquidation. We have asked all ships to come alongside, in preparation for a company announcement. Until then, services from P&O will not be running and we are advising travellers of alternative arrangements.
In a message sent to staff, the company said the company announcement "will secure the long-term viability of P&O Ferries" and has the backing of Dubai-based DP World, which bought the ferry company for £322m in 2019.