Sailings have been cancelled for days and protests are planned after P&O Ferries sacked 800 workers and suspended all of its services.
Staff unions told members to remain on vessels in protest against the move on Thursday, which the firm said was due to huge financial losses.
Security guards boarded ships with handcuffs to remove fired crew, it was alleged.
The convener of a Holyrood committee has called for an “urgent update” from P&O Ferries.
Unions and politicians condemned the mass dismissal, blamed by the company on losses of £100m following the slump in travel because of the pandemic.
Nicola Sturgeon said she spoke directly with the chief executive of P&O and made clear her “utter disgust” at the “appalling treatment of its workers”.
The firm said early on Friday it would not be able to operate services “for the next few days” from Dover to Calais, Hull to Rotterdam, Liverpool to Dublin, and Cairnryan, Scotland, to Larne, Northern Ireland.
The Rail, Maritime and Transport union (RMT) said it was seeking legal advice to challenge the sackings.
It said the UK has seen one of the most “vicious examples of despotic employer behaviour” and one of the most shameful episodes in its recent industrial history.
Trade union Nautilus International, which represents some of those fired, raised safety concerns over the decision to replace 800 seafarers with cheaper agency workers.
It urged the Maritime and Coastguard Agency (MCA) to “make sure the ships are safe” as the new crews are “unfamiliar” with the vessels and routes.
Peter Aylott, director of policy at the UK Chamber of Shipping, which represents the industry, told BBC’s Today he was “very confident that P&O will have put procedures in place to ensure that the individuals that are going to be in control of those vessels will be familiar with the ships, familiar with the systems and will be competent and qualified to operate those vessels in a safe manner”.
Demonstrations are planned at ports in Larne, Dover, Liverpool and Hull.
P&O Ferries said in a statement: “We have made a £100m loss year-on-year, which has been covered by our parent, DP World. This is not sustainable.
“Our survival is dependent on making swift and significant changes now. Without these changes there is no future for P&O Ferries.”
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