Unite said it was keeping a close eye on the car industry
Mini production is to be cut back and the factory closed for a month over Christmas, as the car industry reports falling sales in the UK.
Workers at the BMW-owned factories in Oxford and Swindon have been told the two-week Christmas shutdown is to be extended to four weeks this year.
Production will also stop during the same period at the BMW factory in Hams Hall, north Warwickshire.
Union Unite said the extended shutdown would not affect workers' pay.
It is thought that key workers such as maintenance and engineering staff will remain on site during the closure.
The holiday was going to be from 18 December to 5 January, but will now start on 7 December.
It will affect about 4,700 workers across three shifts on the Oxford site. The Hams Hall plant employs about 1,000 people.
Earlier this week Jaguar Land Rover announced that a voluntary redundancy scheme was to be extended to hundreds of workers.
On Friday, figures from the Society of Motor Manufacturers and Traders showed new car sales in the UK were down 23% from a year ago.
Mini sales in the UK during October were down 40% compared with a year ago. But a spokeswoman for the company said there had been a 12% increase on sales worldwide since last year.
Roger Maddison, national officer at Unite, said: "We understand the very difficult economic conditions car manufacturers are facing.
"We have robust agreements in place to protect our members as much as possible during these difficult times.
"However, we remain vigilant and we are keeping a close eye on developments in the car industry."
BMW spokesperson Angela Stangroom said they were not the only company affected.
"There has been a slowdown in general in terms of car sales in the UK and Mini has been affected by that," she said.
"I think that we have seen a greater reduction in retail sales or cars bought by private individuals.
"Of course, Mini is predominately purchased by those private retail customers."