The Government is being urged to help save jobs at defence giant BAE Systems, which is set to announce cuts at sites making the Eurofighter Typhoon jet.
The announcement, which unions expect to be made on Tuesday, is likely to mainly hit sites at Warton and Samlesbury in Lancashire, where aircraft assembly takes place.
Up to 2,000 jobs may be affected, according to unconfirmed reports.
Labour described the news as "devastating".
A BAE Systems spokesman said: "BAE Systems continually reviews its operations to make sure we are performing as effectively and efficiently as possible, delivering our commitments to existing customers and ensuring we are best placed to secure future business.
"If and when there are any changes proposed we are committed to communicating with our employees and their representatives first."
Nia Griffith, shadow defence secretary, said: "This is devastating news for the workers and their families.
"The men and women who work on the Eurofighter are highly skilled and the potential loss of these jobs would have an appalling impact on them, the local economy and wider supply chains.
"The Government must come forward urgently with a clear plan to secure these jobs at BAE, as well as a proper defence industrial strategy to give the industry the certainty that it needs."
The Prime Minister's official spokesman told a Westminster media briefing on Monday: "No announcement has been made from the company, so I think it would be wrong for me to pre-empt any announcement.
"What I would say is we do have a long track record of working with BAE Systems and its workers and we will continue to do so."
Unite is demanding an urgent meeting with BAE, calling on the company to "come clean" over its plans.
The union warned that the UK's defence capability will be "significantly undermined" and vital manufacturing jobs lost for a generation because of the Government's "continued failure to plan".
Unite estimated that by 2020, 25% of the UK's defence spend will be benefiting American factories and companies such as Boeing and Lockheed Martin.
Assistant general secretary Steve Turner said: "The UK Government can end the uncertainty surrounding the future of thousands of British BAE defence jobs at a stroke by committing to building the next generation fighter jets here in the UK.
"If these job cuts materialise it will significantly undermine our nation's sovereign defence capability and leave us reliant on foreign powers and foreign companies for the successor to the Typhoon and the defence of the nation.
"Once these jobs are gone, they are gone for a generation and with them the skills and ability to control our own defence and manufacture the next generation of fighter jets and other defence equipment in the UK.
"The ripple effects down the supply chain and through our manufacturing communities would be immense too, hitting the workforces in other cutting-edge companies that are involved in the manufacture of one of the best fighter jets in the world, as well as depriving communities of decent well-paid jobs.
"At a time of Brexit, these are precisely the kind of jobs that the UK Government should be protecting. Rather than shipping our defence spend overseas to factories in America and cutting defence, ministers should be investing in jobs, skills and communities by buying British."