The firm, bought by Dubai-based logistics giant DP World in 2019, said it has suspended sailings “for the next few days”.
Workers currently onboard ships were instructed by unions not to leave with chaos at many ports as a result of services being pulled.
A spokesperson for P&O Ferries said the company was currently not a “viable business” and 800 seafarers had been handed immediate severance notices with compensation packages for the “lack of advance notice”.
Coaches carrying agency workers hired to replace the fired staff are parked near ships at ports - although many crew have not yet left their ships, that were moored earlier today.
P&O Ferries said in a statement: “In its current state, P&O Ferries is not a viable business.
“We have made a £100 million 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.”
The company added that, after “seriously considering all the available options”, it has taken the “very difficult but necessary decision” to hand immediate severance notices to 800 seafarers.
RMT members currently on board ships have been instructed by the union not to leave.
RMT general secretary Mick Lynch said: “We are receiving reports that security guards at Dover are seeking to board ships with handcuffs to remove crew so they can be replaced with cheaper labour.
“We are seeking urgent legal action and are again calling for the Government to take action to stop what is fast turning into one of the most shameful acts in the history of British industrial relations.
“If this happens at P&O it can happen anywhere, and we are calling for mass trade union and wider public mobilisation and protest against the company.”
Sacked P&O Ferries workers in Dover have clashed with motorists after blocking a road close to the port.
Dozens of employees who lost their jobs stood on the road with banners and flags saying “Stop the P&O Jobs carve up”.
A lorry driver trying to enter the port began shouting and beeping at those in the road, before being told “we’re not moving”.
One of the protesters told the PA news agency: “I refuse to move from this road, all this service for nothing.
“The police will have to take me away.”
Nicola Sturgeon said the Scottish Government will pay particular attention to the Cairnryan to Larne ferry route.
She tweeted: “I’m deeply concerned at P&O announcement – due to the importance to Scotland of the Cairnryan/Larne route obviously, but also the impact on 100s of workers. Fire & rehire is an appalling practice & offends the basic principle of fair work. @scotgov will be seeking urgent talks.”
Speaking at First Minister’s Questions on Thursday after reports that P&O was about to announce major redundancies, she said: “For Scotland, the relevant issue here is the Cairnryan-Larne route and we will pay particular attention to any implications for that.”
She added: “This will be a seriously worrying time for those who work for P&O.”
The First Minister said she hoped there would not be a “mass-scale fire and rehire situation”.
Labour MP Dame Diana Johnson said people in balaclavas are “taking British crew off these ships”, describing P&O’s actions as “shameful”.
Making a point of order in the House of Commons, the MP for Kingston upon Hull North said: “Members in port constituencies, not least in northern England, will be shocked at the news being reported about DP World, who own P&O Ferries, suspending services this morning, sacking 800 British P&O workers immediately by pre-recorded video message, and them being replaced by agency staff.
“I understand from the RMT union that these agency staff, mainly from overseas, are in buses on the quayside with a security firm, hired by DP World, wearing balaclavas and taking British crew off these ships. This is shameful and it goes against all norms of fair and reasonable behaviour.”
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Mark Dickinson, general secretary of maritime union Nautilus International, said: “The news that P&O Ferries is sacking the crew across its entire UK fleet is a betrayal of British workers.
“It is nothing short of scandalous given that this Dubai-owned company received millions of pounds of British taxpayers’ money during the pandemic.
“There was no consultation and no notice given by P&O.
“Be assured the full resources of Nautilus International stand ready to act in defence of our members.
“We believe it is in our members’ best interests to stay onboard until further notice.”
STUC general secretary Roz Foyer said: “This is a damning, outrageous move from P&O and we offer our full support to the RMT union and all their members.”
P&O Ferries operates four routes: Dover to Calais; Hull to Rotterdam; Liverpool to Dublin; and Cairnryan, Scotland, to Larne, Northern Ireland.
The firm was bought by Dubai-based logistics giant DP World for £322 million in 2019.
DP World was criticised for paying a £270 million dividend to shareholders at the end of April 2020 while P&O Ferries proposed to cut around 1,100 jobs as demand for travel collapsed due to the coronavirus pandemic.
The ferry operator sought a £150 million bailout from the Government, but no offer was made.
Cruise line P&O Cruises is unaffected by the developments as it is a separate business owned by Carnival UK.
The company that would become P&O was founded in 1837 after signing a government contract to transport post by boat between London and the Iberian Peninsula.
The Government will make a Commons statement on P&O ferries at 5pm on Thursday.