P&O Ferries has suspended all sailings on Thursday ahead of a “major announcement” amid fears hundreds of job could be lost.
All vessels have been told to “discharge their passengers and cargo and standby for further instructions”.
First Minister Nicola Sturgeon said she hoped there would not be “a mass scale fire and rehire situation”.
The company, which operates the route between Scotland and Northern Ireland, said it would be unable to run services for “the next few hours”.
P&O is owned by Dubai-based Emirati logistics company DP World.
A statement which appears to be from the ferry operator shared by a Labour MP said it “will be making a major announcement today”.
This will “secure the long-term viability of P&O Ferries”, it read.
A spokesperson denied the company was going into liquidation.
The statement added: “To facilitate this announcement all our vessels have been asked to discharge their passengers and cargo and standby for further instructions.
“This means we’re expecting all our ports to experience serious disruption today.”
P&O Ferries wrote on Twitter: “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.”
RMT, the union, has demanded protection for UK seafarer jobs amid speculation that “hundreds are to be sacked and replaced with foreign labour”.
Responding to reports about the future of P&O, RMT general secretary Mick Lynch said: “We are deeply disturbed by growing speculation that the company are today planning to sack hundreds of UK seafarers and replace them with foreign labour.
“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.”
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.”
UK transport secretary Grant Shapps said his officials will be having “urgent discussions” with the firm.
Shadow transport minister Mike Kane said: “Worrying news has just broken that P&O Ferries have been called to port as DP World, their owners, seek the long-term viability of this ferry company.
“Major disruption is expected. Can the secretary of state update the House on any discussions he’s had with DP World, P&O Ferries of any potential redundancies, and any redundancies that may happen we don’t want those crews replaced by foreign cheap labour.”
Shapps replied in the Commons: “I am concerned with this news which is breaking on P&O Ferries. 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.”
Commons Speaker Sir Lindsay Hoyle said he would allow an “emergency statement” from the UK Government on Thursday if needed.
P&O is the leading ferry operator on the Dover to Calais crossing, the main sea link from the UK to Europe, and also sails from Hull to Rotterdam, Liverpool to Dublin and Cairnryan in Scotland to Larne.