No 10 has backed calls for the chief executive of P&O Ferries to quit his post, but refused to guarantee the company would be prosecuted under planned changes to the law.

Asked if the prime minister supported Grant Shapps’s call for the P&O chief executive to quit, a No 10 spokesperson told reporters on Friday: “Yes”.

The spokesman, however, refused to guarantee that the planned law changes would see P&O prosecuted — despite Boris Johnson making that promise in the Commons.

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In a series of tweets, the RMT union – representing most of the sacked P&O Ferries seafarers – has questioned the safety of ships operated by new crew.

The union said: “UK seafarers with decades of experience onboard P&O Vessels have been replaced by crews with no knowledge of safe systems of work, risk assessments and intimate knowledge of a vessels layout and emergency procedures.

“P&O’s own study from 2012 showed moving to longer shift patterns of 2 weeks on and 2 weeks off created dangerous increases in crew fatigue. The same ships are now 10 yrs older & P&O want to move from 1-week on to 8-week on contracts!

“Crew from Ukraine, Latvia, Philippines & India raise serious concerns over Standard Maritime Communication Phrases onboard, particularly during an emergency situation.”

In a select committee hearing at Parliament on Thursday, Katy Ware, director of UK Maritime Services, told MPs: “You have my absolute assurance that we will go into absolute detail to ensure that those crew on board are familiarised, qualified, trained, and that we are satisfied they can operate those ships safely.”

She was responding after Mark Dickinson of Nautilus International, who represents sacked officers, said of the Dover-Calais route: “It’s an incredibly dangerous situation, and you need skilled, qualified, experienced maritime professionals.”

Ms Ware said: “Those vessels will not leave until we are satisfied that they are safe to operate.”

One of P&O Ferries’ rivals on Channel crossings, Brittany Ferries, has welcomed the UK’s first ferry powered by liquid natural gas (LNG) to Portsmouth.

Salamanca is the first of four LNG ferries to sail from Portsmouth to France and Spain. Two of them will be hybrid-electric vessels.

Christophe Mathieu, chief executive of Brittany Ferries, said: “LNG-powered ships like Salamanca are a clear statement of our commitment to the future and to fleet renewal.

“They are cleaner vessels, significantly cutting air quality emissions like soot and sulphur.”

The new ship will make two round trips to Bilbao each week, as well as a weekly voyage to Cherbourg in France.

All Brittany Ferries staff are employed according to French labour laws.

The RMT union, which represents most of the seafarers abruptly sacked by P&O Ferries, plans a demonstration in Liverpool on Saturday to call for their reinstatement.

The union calls it the “Stop the P&O jobs massacre demonstration”.

It will begin at the Seafarer Centre in Waterloo, Liverpool, at 11.30am.

Meanwhile the Scottish TUC has a demonstration at 11am on Monday 28 March outside Clyde Marine Recruitment in Govan, Glasgow.

Clyde Marine Recruitment is one of the agencies that P&O Ferries is using for replacement crew. The Scottish TUC tweeted: “Bring your flags and banners, all welcome!

“It’s time to target P&O’s supply chain and any company complicit in P&O’s scandalous decision to dismiss all UK seafarers.”

Transport secretary Grant Shapps said he has spoken to P&O workers who have already accepted other jobs.

When asked about how government plans to change the law would help sacked seafarers, Mr Shapps told Good Morning Britain some former employees had already been “paid off for their silence” by the company and had been offered jobs elsewhere.

He said: “I actually happen to know because I’ve been speaking with some of them that some of them have already taken up a job, so they may not get those particular ones back.

“I suspect they’ll get some of them back, although I imagine they’ll have a slightly rather jaded view of their employer as P&O, particularly if the current boss stays in place, which is why I’ve said this morning: ‘You’ll have to go.’

“But we will make sure the laws are changed to stop them using loopholes like flagging their ships in Cyprus to avoid and evade British law and not give notice of what they were doing, and not talk to the workers and the unions.

“They are completely outrageous and we will not let it stand.”

Reporting by agencies.

The MP for East Leeds, Richard Burgon, has called for a repeal of the anti-trade union laws in the wake of P&O’s controversial sacking of 800 seafarers this month.

“The P&O case must be a line in the sand. It’s time to repeal all the anti-trade union laws. Give workers a proper chance to fight back against terrible bosses,” wrote Mr Burgon on Twitter.

Grant Shapps has said that P&O Ferries is attempting to “buy silence” from its staff over the mass sackings last week.

The transport secretary told BBC Radio 4’s Today programme: “What they’ve done is paid off - or attempted to pay off - their staff with higher redundancy payments... and therefore buy their silence.

“We cannot have a situation where laws are being creatively used, or abused in this case, in order to get around what Parliament has very clearly intended to do. We have a (National) Minimum Wage Act.

“They flagged, for example, their ships in Cyprus in order not to have to provide us with notice of this. I mean, it’s completely irresponsible. It’s completely wrong.”

In Thursday’s committee session at Parliament, Peter Hebblethwaite, chief executive of P&O Ferries, provided an insight into the planned cut-price operation for the firm’s operations, particularly on the key Dover-Calais route.

Seafarers live on board the ships and generally work 12-hour shifts.

Crew will work on board the ship for two weeks on/two weeks off, rather than the current one week on/one week off.

At an average wage of £5.50 per hour, they will earn £66 for each 12-hour day. They are provided food and accommodation on board. That is £41 less than they would earn at the National Minimum Wage.

Mr Hebbelthwaite said he had a basic salary of £325,000, with short-term and long-term incentive bonus schemes.

Assuming a 40-hour week and four weeks’ holiday, his hourly rate is £169.

Transport secretary Grant Shapps has said the government “can’t directly” revoke P&O’s licence when asked why the company was still operating after having broken the law.

He told BBC Breakfast he had instead asked the Maritime Coastguard Agency to carry out “very detailed inspections”.

The minister said if new crews were being paid under national minimum wage and found to be unfamiliar with equipment, the ships would be deemed unsafe to sail.

Presenter Sally Nugent asked him: “You can freeze the assets of Russian oligarchs but you can’t for example revoke P&O’s licence?”

He replied: “At the moment we can’t directly, but I’ve already asked the Maritime Coastguard Agency to carry out very very detailed inspections of the ships.

“We do have some mechanisms. There are a whole load of other things happening including letters going out to the ports from which they sail, then, as I mentioned, different types of legislation will be forthcoming, so we are going to make sure P&O have to U-turn on this.”

Reporting by agencies.

Union leaders are to hold talks with P&O Ferries on Friday to demand the reinstatement of sacked seafarers.

The Rail, Maritime and Transport union (RMT) said it will be meeting with the company following the huge outcry over the dismissal of almost 800 workers.

More protests are being planned this weekend as unions keep up the pressure on the company and the government to take action.

P&O’s chief executive admitted to MPs on Thursday that the law had been broken in the way the seafarers had been treated.

RMT general secretary Mick Lynch said: “RMT will be holding talks with P&O today to demand the reinstatement of our sacked seafarers.

“We welcome the massive public and political support for our campaign.

“P&O Ferries need to change course and reinstate these loyal key workers.”

Reporting by agencies.