Sailings have been suspended by P&O Ferries as the company confirmed it has made 800 employees redundant.
The ferry operator had said in an internal statement it will make "a major announcement" which will "secure the long-term viability of P&O Ferries".
Following hours of speculation, a spokesman said: "P&O Ferries plays a critical role in keeping trade flowing, supply chains moving, and connecting families and friends across the North and Irish seas and the English Channel. We have been at the heart of this service for years and we are committed to serving these vital routes."
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"However, in its current state, P&O Ferries is not a viable business. We have made a £100m 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.
"These circumstances have resulted in a very difficult but necessary decision, which was only taken after seriously considering all the available options.
"As part of the process we are starting today, we are providing 800 seafarers with immediate severance notices and will be compensating them for this lack of advance notice with enhanced compensation packages.
"In making this tough decision, we are securing the future viability of our business which employs an additional 2,200 people and supports billions in trade in and out of the UK.
"And we are ensuring that we can continue serving our customers in a way that they have demanded from us for many years."
Urging members to stay onboard until further notice, as it is deemed in their best interests, Nautilus International general secretary Mark Dickinson 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 consolidation and no notice given by P&O. Be assured the full resources of Nautilus International stand ready to act in defence of our members."
East Hull Labour MP Karl Turner described P&O’s actions as an "utterly deplorable predatory practice taking full advantage of the gap in the legislation".
"I’ve been warning the Government about this for long enough," he said, adding a new foreign crew was waiting to board Pride of Hull in the city’s docks. It sails to Rotterdam, the only remaining route, with the firm having ditched Zeebrugge during the pandemic.
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.
"We cannot - and will not - permit hundreds of workers to be sacked on the spot to be replaced by cheaper labour whilst P&O scramble to remain viable.
"We need to see an urgent statement from both the UK Government and the Scottish Government on how they intend to halt this scandalous misuse of employer power."
Ferries from Hull, Liverpool, Cairnryan in Scotland and Dover have been affected.
P&O Ferries, which transports passengers and freight, is owned by Dubai-based logistics giant DP World.
It operates these four routes: Dover to Calais; Hull to Rotterdam; Liverpool to Dublin; and Cairnryan, Scotland to Larne, Northern Ireland.
Sailings between Hull and Zeebrugge, Belgium, were axed in January 2021.
Following the coronavirus outbreak, P&O Ferries warned in May 2020 that around 1,100 workers could lose their jobs as part of a plan to make the business "viable and sustainable".
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.
