Rolls-Royce is moving production of fan blades from its 80-year-old factory in Barnoldswick, Lancashire, to Singapore from Friday, unions have said.
There are fears the move could also result in the outright closure of the plant, with just 150 people left working there.
Unite said staff have been told that “on the basis of health and safety” the site will close from Friday for an extended Christmas break, with staff furloughed on 80% of pay.
They were also told work currently undertaken at Barnoldswick will be transferred to Japan, Singapore and Spain.
Rolls Royce told Mirror Online that the closure move was not permanent and changes did not amount to offshoring work.
Unite members have been carrying out targeted strike action since 6 November as part of a campaign to secure the future of the historic factory. This was set to end on 24 December 24.
In August Rolls-Royce said it planned to offshore the work on its Trent jet engine blades from Barnoldswick to a site in Singapore, with the loss of 350 jobs.
Unite regional officer Ross Quinn said: “The decision to lock workers out of Barnoldswick before Christmas and to immediately offshore work at the factory demonstrates that Rolls-Royce has no intention of negotiating or consulting its loyal workers on its plans.
“Workers at Barnoldswick, who take huge pride in their work, began targeted industrial action as a last resort in order to ensure the future of the historic factory."
He added: “We remain committed to finding a resolution and call on the company to meet with Unite immediately before they do irreparable damage to this workforce and community.”
A Rolls-Royce spokeswoman said: "We always have a shutdown of our UK facilities over Christmas.
"In Barnoldswick we are having to carry out that shutdown early because the staff who help us close the site safely and securely are planning to be on strike from the end of this month.
"We will be furloughing employees - on 80% of their normal pay - but will not be claiming this money back from the UK Government.
"Due to the length of the shutdown which the strike action has caused, we will have to source parts that would normally be made in Barnoldswick from other parts of our supply chain but that is not the same as moving work overseas."
Barnoldswick began producing the first jet engines, invented by Frank Whittle in the 1940s, as part of the war effort.
The plant was was first owned by Rover, but taken over by Rolls-Royce in 1943.
The first jet engines made there were for Gloster Meteors, Britain's first fighter jets, which flew in 1943.
