Hundreds of people have demonstrated against P&O Ferries’ sacking of 800 staff at ports across the UK, with some chanting: “P&O, shame on you.”

Protesters carrying banners and placards gathered in Dover, Liverpool, and Hull as part of what they called the “fight for justice” for the seafarers dismissed without notice earlier this month.

The demonstrations took place as the European Causeway, a ship operated by the company, remained detained in Larne, County Antrim, for being “unfit to sail”.

The National Union of Rail, Maritime and Transport Workers (RMT) promised more protests.

General secretary Mick Lynch said: “All the protests today were well attended with hundreds turning out in Liverpool, Hull, and Dover to support the P&O workers in their fight for justice.”

He said the demonstrations “show that the public is on our side regarding the P&O sackings”.

Ministers, he added, “need to bring in emergency legislation and prevent P&O from sailing with under trained, super exploited, agency crews”.

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Two more P&O ships face checks next week, Sky News understands.

A spokesperson for the Maritime and Coastguard Agency (MCA) said the European Causeway “has been detained due to failures on crew familiarisation, vessel documentation and crew training.

“The vessel will remain under detention until all these issues are resolved by P&O Ferries. Only then will it be reinspected.”

The MCA said there were no passengers or freight on board the European Causeway vessel when it was detained.

The detention of ships is based on concerns over their safety and to prevent them going to sea.

A P&O Ferries spokesman said: “European Causeway has undergone an inspection by the MCA in Larne, during which it was deemed 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.”

The Trades Union Congress (TUC) tweeted a video they said showed P&O dockers in Rotterdam refusing to load freight on to a ferry set for Hull “in solidarity with the 800 seafarers illegally sacked by P&O”.

Trade union workers gathered at Dublin Port outside the P&O terminal to send support from across the Irish Sea to P&O staff.

Meanwhile, Labour has written to Business Secretary Kwasi Kwarteng asking whether the government will seek the removal of P&O Ferries’ chief executive Peter Hebblethwaite as a director under the Company Directors Disqualification Act 1986.