Protests are taking place at UK ports over the sacking of hundreds of seafarers, as calls grow for a P&O Ferries boss to quit.

The demonstrations come after a ship operated by the ferry firm was detained for being “unfit to sail”.

The European Causeway vessel has been held at the port of Larne in Northern Ireland due to “failures on crew familiarisation, vessel documentation and crew training”, the Maritime and Coastguard Agency (MCA) said.

Transport Secretary Grant Shapps said he will not compromise the safety of P&O vessels and insisted that the company will not be able to rush training for inexperienced people.

In a move that sparked widespread outrage, the company sacked almost 800 seafarers earlier this month, and plans to replace them with agency staff on cheaper salaries.

A spokesperson for the Maritime and Coastguard Agency said: “We can confirm that the European Causeway has been detained in Larne.

“It 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.”

On Saturday, protesters are gathering in Liverpool, Dover and Hull as the RMT union urged a stop to the “P&O Jobs Massacre”.

People chanted “P&O, shame on you” as they marched along a street in Liverpool, with more than 100 turning out, according to the union.

In Dover there were similar scenes as crowds carried banners and placards calling for an end to the “P&O jobs carve up”.

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

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

The rally, organised by trade union Siptu, was attended by the new Irish Labour Party leader Ivana Bacik, and organiser Jim McVeigh told the PA news agency that the rally was intended to “send solidarity”.

He added: “The best thing we could do was to have a rally at Dublin Port outside P&O so they know our concerns.”

UK 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.

In a letter, the party accused the Government of “sitting on their hands” rather than taking action to hold P&O to account, adding that the “toothless response risks giving the green light to exploitation”.

Shadow transport secretary Louise Haigh said the “shameful misconduct of P&O Ferries has ruined livelihoods” as she called for the sacked workers to be reinstated and for Mr Hebblethwaite to be “barred” as a director for his role in the crisis.

Prime Minister Boris Johnson backed Mr Shapps’ call for Mr Hebblethwaite to quit.

In Larne, the local mayor of the Mid and East Antrim Council, William McCaughey, said they would support the reinstatement of the staff immediately.

“It is ridiculous what P&O has done to the staff, we in Larne would be very keen to see staff reinstated, it is the least that P&O could do,” he told the PA news agency.

Alliance East Antrim MLA Stewart Dickson welcomed the impounding of the ferry as a safety measure.

“It’s not like the crew of an airplane getting off one easyJet and getting on to the next one where the controls are the exactly the same, and everything is in the same place,” he told PA.

“No two ships are the same, and you cannot just fly a crew in and expect them to be able to sail a ship.

“Every control will be in a different place, but particularly all those health and safety drills that have to be gone through, everything from lifeboat stations to how each item of equipment operates.

“It seemed to me it was going to be very difficult for staff to be able to take on that role in such a short period of time.

“I am absolutely delighted they have (impounded the ship). This isn’t vengeance against P&O, it’s about passenger safety and the safety of the crew as well.

The RMT union said it welcomed the detention of the European Causeway and it demanded the Government “seize the entire fleet” of P&O vessels.