The Maritime and Coastguard Agency confirmed the European Causeway had been held in Larne.

The MCA said there were “failures on crew familiarisation, vessel documentation and crew training”. P&O Ferries sacked 800 staff without notice eight days ago.

The firm said it would make the changes needed to return the ship to service.

Unions have also raised fears over a lack of training of new crew following the firm replacing their members with workers whose average hourly rate of pay would be £5.50, less than the UK minimum wage.

The European Causeway sails between Larne and Cairnryan in Scotland. P&O Ferries told customers on Twitter that its services on this route were suspended, adding: “It is no longer possible for us to arrange travel via an alternative operator on this route.

“For essential travel, customers are advised to seek alternatives themselves.”

On Friday, Prime Minister Boris Johnson backed Transport Secretary Grant Shapps’s calls for the boss of P&O Ferries to resign.

Peter Hebblethwaite admitted to MPs that he broke the law by not consulting workers ahead of the job cuts – but said he would do the same again if he had to.

There were no passengers or freight aboard the European Causeway when it was impounded in Larne, the MCA confirmed.

It said the vessel would remain under detention until all issues were resolved by P&O Ferries.

Mr Shapps wrote on social media that the ship’s detention followed an instruction from him to the MCA to inspect “all P&O vessels” before they entered back into service, adding: “I will not compromise the safety of these vessels, and P&O will not be able to rush inexperienced crew through training.”

Labour’s Louise Haigh called for the sacked workers to be reinstated and for Mr Hebblethwaite to be “barred” as a director.

“The shameful misconduct of P&O Ferries has ruined livelihoods, and is harming the UK’s key shipping routes,” the shadow transport secretary wrote on Twitter.

P&O Ferries said in a statement that an MCA inspection had deemed the European Causeway “not sufficiently ready for entry into operation”.

“We shall review the findings, make any changes required and continue to work closely with the MCA to return the ship to service.”

Meanwhile P&O Ferries services between Dover and Calais remain cancelled. The firm said it would organise an alternative carrier for passengers.

Saturday evening’s service from Rotterdam to Hull is also suspended, the firm said – but the evening service from Hull to Rotterdam is expected to sail on time.

The general secretary of the RMT union, Mick Lynch, said that the seizing of the European Causeway suggested the firm was not “fit and proper to run a safe service after the jobs massacre”.

He called for the sacked crews to be reinstated “to get these crucial ferry routes back running safely”.

Maritime trade union Nautilus International said the importance of a well-trained crew “cannot be overstated”.

General secretary Mark Dickinson said this was “even more the case” for an operator like P&O Ferries, which carries up to 2,000 passengers a day and follows extremely tight schedules along one of the busiest shipping lanes in the world.

“The consequences can be fatal when commercial pressure takes precedence over safety concerns in the ferry sector,” he said.