Jaguar Land Rover is to shut down production for a week because of uncertainties around Brexit.
It will affect thousands of staff at Castle Bromwich, Solihull and Wolverhampton in the West Midlands, and Halewood on Merseyside, from Monday.
The shutdown is in addition to a scheduled closure the following week for Easter.
Unite the union said the move was agreed in January when the UK was due to leave the EU on 29 March.
The company said it needed more certainty around Brexit, and warned that a “no-deal” Brexit would cost it more than £1.2bn in profit each year.
Mick Graham, Unite’s convenor at Solihull, said: “We had to make some plans to protect the business as best we could and we started talking about this in January.
“We knew we had to take reactive action to mitigate the potential effect of a bad Brexit or no-deal Brexit.
“Suppliers need notice to get their parts across to us… It was a prudent thing to do.”
The car maker, which employs just under 39,000 workers, also announced in January it was cutting 4,500 jobs, with the majority coming from its UK workforce.