It's being widely reported staff at P&O Ferries are going to be made redundant
It operates four routes including Hull to Rotterdam
P&O Ferries has suspended sailings amid speculation it is preparing to sack hundreds of workers.
The ferry operator said in an internal statement it will make "a major announcement" which will "secure the long-term viability of P&O Ferries".
Rail, Maritime and Transport (RMT) union 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 on board 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."
A crew member, who wished to remain anonymous, told KentOnline: "As of effect from today all our contracts have been terminated and we've all been made redundant."
Labour MP Karl Turner posted a photograph of a coach which he stated contained "new foreign crew waiting to board the Pride of Hull" at King George Dock, Hull.
RMT members are "sitting in onboard the vessel", so the new crew "will not be boarding her", he wrote.
He added: "We understand that both current officers and ratings are to be sacked."
In its internal statement, P&O Ferries said: "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."
Transport Secretary Grant Shapps told the House of Commons he is "concerned" by the situation.
He said: "I understand they have temporarily paused their operations and that's causing disruption at the short straits - Calais-Dover - as well as some other ports.
"I'm working with the Kent Resilience Forum and I've just instructed them to become intricately involved, and other partners in this, and we'll be taking steps later today - including ensuring that my officials will be having urgent discussions with P&O about the situation, particularly of concern for their workers."
P&O Ferries, which transports passengers and freight, has nearly 4,000 employees.
It was bought by Dubai-based logistics giant DP World for £322 million in 2019.
It operates four routes: Dover to Calais; Hull to Rotterdam; Liverpool to Dublin; and Cairnryan, Scotland, to Larne, Northern Ireland.
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".
Stormont Assembly members whose constituency incorporates the port of Larne expressed concern about the potential implications for the route to Cairnryan.
East Antrim MLA John Stewart, who has relatives who work at the port, said staff were uncertain about what would be announced.
"The information is being nothing more than drip-fed to them," the Ulster Unionist MLA told BBC Radio Ulster.
He said it is a "worrying" development.
"For the staff, for Larne port itself, particularly for the local effect here, and also for businesses and passengers that regularly use that service, it's an invaluable local service over to Cairnryan and I think this news will be deeply worrying for all of them, especially in the absence of any concrete information as to what the long-term strategy is going to be."
The company that would become P&O Ferries was founded in 1837 after signing a government contract to transport post by boat between London and the Iberian Peninsula.
Cruise line P&O Cruises is unaffected by the developments as it is a separate business owned by Carnival UK.