The decision by P&O Ferries to sack 800 seafarers with no notice and replace them with cheaper agency workers has raised safety fears.
Trade union Nautilus International, which represents some of those fired, has 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.
P&O Ferries told passengers sailings will be suspended "for the next few days".
READ MORE: P&O Ferries suspends sailings and makes 800 employees redundant
Nautilus International general secretary Mark Dickinson said it was "an intensely worrying situation" as sailing ships across the Channel is "like walking across a six-lane motorway at rush hour", the PA news agency said.
He told BBC Radio 4's Today programme: "There are serious safety concerns, which is why the company cannot reintroduce services with the lower-paid agency crew that they've recruited via this company called International Ferry Management of Malta."
Mr Dickinson said the MCA must be "absolutely clear and confident that those new crew, unfamiliar with the vessels, unfamiliar with the routes, with the berths" can operate the ships safely.
He added: "We've written to the Maritime Coastguard Agency and we hope and we pray that they will do their job."
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Peter Aylott, director of policy at the 海角视频 Chamber of Shipping, which represents the industry, told 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 Dover, Liverpool and Hull as unions and politicians condemned the mass dismissal, blamed by the company on losses of £100 million following the slump in travel because of the pandemic.
The Rail, Maritime and Transport union (RMT) said it was seeking legal advice to challenge the sackings.
Before suspending sailings, P&O Ferries operated four routes: Dover to Calais, Hull to Rotterdam, Liverpool to Dublin, and Cairnryan, Scotland, to Larne, Northern Ireland.
It advised those already at Dover and Calais to make their way to the check-in booths for Danish firm DFDS, but there were no such instructions for those at Hull, Rotterdam, Liverpool, Dublin, Cairnryan or Larne.
Announcing the decision on Thursday, the ferry operator, bought by Dubai-based logistics giant DP World in 2019, insisted the decision to cut jobs was "very difficult but necessary" as it was "not a viable business" in its current state.
It 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."