P&O Ferries, which runs sailings from Scotland to Northern Ireland, has suspended sailings ahead of a "major announcement" but insisted it is "not going into liquidation".
In an internal company statement, the ferry operator said it will be "making a major announcement today" which would "secure the long-term viability of P&O Ferries".
All sailing staff working for P&O Ferries have been made redundant, it has been reported. ITV News said the redundancy is with 'immediate effect'.
It reported that the ferries will be kept running with agency staff. P&O sailing staff will be able to apply for work with the agency.
ITV's Shehab Khan said on twitter: "Understand that all sailing staff at P&O Ferries have been made redundant with immediate effect. The company will use an agency to keep their ferries running and current staff will be able to apply to the agency for work."
All staff at P&O Ferries made redundant with immediate effect, with company to use agency staff to sail their ships
The statement added: "To facilitate this announcement all our vessels have been asked to discharge their passengers and cargo and stand by for further instructions.
"This means we're expecting all our ports to experience serious disruption today."
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Rail, Maritime and Transport union (RMT) general secretary Mick Lynch said there is "growing speculation that the company are today planning to sack hundreds of UK seafarers and replace them with foreign labour".
He added: "We have instructed our members to remain onboard and are demanding our members across P&O's UK operations are protected, and that the Secretary of State intervenes to save UK seafarers from the dole queue."
P&O Ferries, which transports passengers and freight, is owned by Dubai-based logistics giant DP World.
It operates Cairnryan in Dumfries and Galloway to Larne, Northern Ireland, as well as Dover to Calais; Hull to Rotterdam; and Liverpool to Dublin.
Sailings between Hull and Zeebrugge, Belgium, were axed in January 2021.
Following the coronavirus outbreak, P&O Ferries warned in May 2020 that around 1,100 workers could lose their jobs as part of a plan to make the business "viable and sustainable".
A spokesman for the firm 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."
The company that would become P&O was founded in 1837 after signing a government contract to transport post by boat between London and the Iberian Peninsula.
