Defence giant BAE has confirmed it is considering closing one of its major shipyards in a move that could threaten up to 3,000 jobs.
BAE's UK chief executive Nigel Whitehead told The Sunday Telegraph a decision was expected by the end of the year.
The future of its three major bases - one at Portsmouth and two in Glasgow, at Govan and Scotstoun - has been under threat after BAE launched a review of its maritime operations at the start of the year.
It is believed Portsmouth is most at risk and a closure could put about 3,000 jobs at risk.
Mr Whitehead told the newspaper that plans for a "reduction in footprint" could see "the cessation of manufacturing at one of the sites".
"We will be making decisions this year," he added.
The blow comes after last month's collapse of the planned mega-merger between BAE and Airbus parent EADS.
The two groups called time on the tie-up - which would have created the world's biggest defence and aerospace group with 220,000 staff worldwide and combined sales of £60 billion - after political hurdles proved insurmountable.
BAE said it was in contact with the Ministry of Defence as it reviews its shipbuilding future.
It added: "We continue to work closely with the Ministry of Defence to explore all possible options to determine how best to sustain the capability to deliver complex warships in the UK in the future.
"This work is ongoing and we are committed to keeping our employees and trade unions informed as it progresses."
The group employs about 3,500 staff across its Glasgow shipyards and nearly 5,000 at Portsmouth, although less than half are directly involved in shipbuilding.
BAE has been coming under pressure from government spending cuts and it is feared there will not be enough work to keep all three sites profitable, with a gap expected between the completion of work on the Queen Elizabeth class aircraft carriers and the start of the Type 26 Global Combat Ship programme for the Royal Navy.
London-headquartered BAE is the UK's main defence supplier, making Astute nuclear-powered submarines, aircraft carriers and fighter jets.
But it has been hit by cuts in government military spending in recent years, which led to a 14% fall in annual sales and left 2011 profits 7% lower at £2 billion.
Jim Murphy MP, shadow defence secretary, said: "This is worrying for everyone involved.
"There must be clarity from the UK Government over the future of these yards and workforces.
"Scotland has such a proud shipbuilding history and it should be a part of our future as well.
"The Tory government plans are adding to the worries but there is one certainty which is that the SNP proposals would sink Scottish shipbuilding.
"We know for a fact independence would close the Scottish shipyards.
"The rest of UK would become a foreign country to Scotland and the UK Royal Navy has not built a warship in a foreign land in living memory.
"The Royal Navy order book keeps Scottish yards afloat but independence would see orders dry up. Thousands of jobs are put at risk by Nationalists' plans."
A Scottish Government spokesman said: "BAE Systems is an important employer in Scotland.
"We will monitor this situation closely and work with BAE with the aim of ensuring any future plans protect the interests of employees and shipbuilding in Scotland."
An SNP spokesman said: "For Labour to try and make political attacks when people's jobs may be in danger shows how badly warped their priorities have become.
"Scotland's defence sector has the engineering capability and skills base to thrive in an independent Scotland, as was made clear last week by defence expert Ian Godden.
"With BAE considering closing a shipyard, talking down the strength of Scotland's shipyards is completely irresponsible and absolutely the last thing that Labour should be doing."