The PSA Group will reduce the Vauxhall plant in Ellesmere Port, north-west England, to a four-day working week from 23 March.
A letter sent to employees and then seen by the Liverpool Echo newspaper revealed the plans. Vauxhall has since acknowledged them in a statement but claims employees would work "extended hours" on those four days to match the current five-day week.
The statement doesn't rule out job cuts or restructuring in the near future, with Vauxhall saying it will "study the result of this decision and discuss with workforce representatives and trade unions whether there are any impacts".
A statement from Ellesmere Port plant director Mark Noble said the decision was taken due to a fall in demand for the Astra Sports Tourer estate, which is built in both Vauxhall and Opel forms at the plant. Both brands sold less than 137,000 Astras last year, not far off half the volumes achieved in 2016.
Ellesmere Port has seen its workforce dramatically reduce since PSA took over the twin brands from General Motors. It's feared that more jobs could be eliminated as a result of the reduced working week.
Noble claimed the decision "will not influence negatively whether Ellesmere Port is allocated to the new Astra". However, workers still fear that it won't wont be retained as the model's second production base, alongside Rüsselheim in Germany, when the next-generation model is introduced in 2021.