Defence giant BAE Systems is to axe 1,775 jobs across its naval ships business and end shipbuilding at one the country's most historic yards.
The firm said 940 jobs will be lost in Portsmouth, on the south coast, and a further 835 in Glasgow, Rosyth and Filton, near Bristol.
Shipbuilding operations will end in Portsmouth in the second half of next year, but an engineering team will be retained to support the new Type 26 warships, which will be built in Glasgow.
BAE said it remained committed to continued investment in Portsmouth as the centre of its maritime services and high-end naval equipment and combat systems businesses.
BAE said it was being hit by a "significant" reduction in workload following the peak of activity on the current aircraft carrier programme.
The grim news was given to workers at a series of meetings at 11am across the affected sites, before they were allowed to go home for the rest of the day.
David Hulse, GMB national officer and chair of the Confederation of Shipbuilding and Engineering Unions' (CSEU) shipbuilding national committee, said: "Following today's announcement from BAE Systems, we are able to confirm that no shipyard will be closing even though there are substantial job losses in the pipeline.
"There is no doubt that this is a devastating day for the UK shipbuilding industry and the company will have justify to us the job losses planned.
"We have arranged a two-day meeting with the company at Farnborough next Monday and Tuesday that will be attended by officers and shop stewards from all the yards and all the unions. This meeting will examine in detail the business case and all aspects for scheduling work in the yards to complete building the carriers, starting work on the Type 26 ships and any other work."
Hugh Scullion, general secretary of the CSEU, said: "The CSEU has been assured that further discussions will take place in the coming weeks with BAE over the future of its marine division, which has huge strategic importance for the UK's defence industry.
"Getting an agreement which avoids the need for compulsory redundancies will be central to our discussions with the company. The CSEU will also make it a priority to protect the future of the UK shipbuilding industry by securing investment to ensure the industry doesn't just survive but prospers in the future. "
BAE said: "Following detailed discussions about how best to sustain the long-term capability to deliver complex warships, BAE Systems has agreed with the UK Ministry of Defence that Glasgow would be the most effective location for the manufacture of the future Type 26 ships.
"Consequently, and subject to consultation with trade union representatives, the company proposes to consolidate its shipbuilding operations in Glasgow with investments in facilities to create a world-class capability, positioning it to deliver an affordable Type 26 programme for the Royal Navy.
"The cost of the restructuring will be borne by the Ministry of Defence. The implementation of these restructuring activities will sustain BAE Systems' capability to deliver complex warships for the Royal Navy and secure the employment of thousands of highly skilled employees across the UK.
"In 2009, BAE Systems entered into a terms of business agreement (ToBA) with the Ministry of Defence that provided an overarching framework for significant naval shipbuilding efficiency improvements in exchange for commitments to fund rationalisation and sustainment of capability in the sector.
"The agreements announced today, together with an anticipated contract for the design and manufacture of the Type 26 Global Combat Ships programme, will progressively replace that ToBA."
Steve Murphy, general secretary of construction workers union Ucatt, said: "Once again workers have found out that their jobs are under threat through the media. It is highly distressing and extremely unfair to treat a loyal workforce in such a way.
"This is a highly skilled workforce, so it is absolutely essential that the Government and BAE Systems minimises job losses, guarantees the long-term future of all the shipyards and continues to invest in training the workers of the future."
Unite national officer for shipbuilding Ian Waddell said: "This is a very worrying time for the workforces and their families as the work on the two carriers comes to a conclusion.
"Unite will be working very hard to retain the maximum number of jobs at both Portsmouth and in Scotland.
"It is a huge blow to Britain's manufacturing and industrial base, with many highly skilled workers faced with losing their jobs.
"We will have to examine the BAE business case in detail to see how we can secure a future for the workforces at both Portsmouth and in Scotland. We believe that, if this is approached in a constructive and innovative way, it can be achieved.
"The seeds for this situation were sown in the 1980s when the Thatcher government used European structural funds to close shipyards, rather than funding investment that would have allowed Britain to compete in the global marketplace for shipbuilding orders against the likes of South Korea."
Gerald Vernon-Jackson, leader of Portsmouth City Council, said: "I condemn the decision to shut down the last remaining shipyard in England with the capability to build advanced surface warships.
"This decision is bad for Portsmouth, with the loss of many highly-skilled jobs, but it's also bad for the defence of the UK and for the Royal Navy.
"The remaining yards with the capability to build advanced warships are in Scotland, and the referendum on Scottish independence is less than one year away. Ministers have put the defence of the UK and the future of the navy at real risk.
"We will work as hard as we can to protect jobs in Portsmouth. Portsmouth remains the home of the Royal Navy, with more than 10,000 jobs remaining in the dockyard."
Defence Secretary Philip Hammond will address MPs on the future of shipbuilding at 12.30pm, said Downing Street.
Prime Minister David Cameron's official spokesman told a regular Westminster media briefing: "These are decisions taken in the UK's national interest."
Asked whether the desire to encourage Scots to vote No in next year's independence referendum had played any part in the decision, the spokesman said: "This is a Government that always takes decisions based on the national interest.
"This decision was taken with a view of how we have the best-equipped, best-maintained Royal Navy. That is the basis on which it was taken."
Asked whether any contingency plans had been made to preserve shipbuilding capacity in the rump UK in the case of Scotland voting for independence, the spokesman said: "The basis on which we proceed is on the basis that we are confident that the case (for Scotland to stay in the UK) will be successfully made."
The consequence of the decision was that "we have Portsmouth as the home of the Royal Navy fleet... and world-class shipbuilding of Royal Navy ships continuing on the Clyde", said the spokesman.
The MoD announced it is to commission three new ocean-going offshore patrol vessels for the Royal Navy, to be built by the BAE yards on the Clyde in Scotland.
The deal provides work for the company between the completion of the current aircraft carrier contract and the start of the Type 26.
Mr Hammond also announced that more than
