Jaguar Land Rover has begun its week-long factory shutdown as part of its plans for Brexit, on the day the company posted lower sales in Europe and China.

JLR’s four main UK manufacturing sites – at Castle Bromwich, Solihull and Wolverhampton in the West Midlands, and Halewood in Merseyside – which employ 18,500 people, are closed from Monday until Friday.

The production shutdown at Britain’s biggest carmaker is in addition to its usual Easter closure, which runs from next week until 23 April. The extension was agreed with staff in January to prepare for potential Brexit disruption, when the UK’s scheduled departure date from the EU was 29 March.

Theresa May is locked in talks with Labour to come up with a plan she can take to an emergency European council summit on Wednesday, at which EU members will decide whether to grant the UK a further extension. Otherwise, the country will crash out of the EU without a deal on Friday.

Workers at JLR’s UK factories will be paid during the production shutdowns but will have to make up the hours at a later date. The company has also started cutting 4,500 jobs from its 40,000 global workforce, affecting mainly management roles in the UK.

Mick Graham, Unite’s plant 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.

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“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.”

Along with other major carmakers, JLR has been stockpiling vehicle components, but can prepare for only days of disruption, rather than weeks, as it uses 25m separate parts per day. BMW brought forward its annual maintenance to April and warned that a no-deal Brexit might force it to stop making the Mini at its Cowley plant near Oxford.

Britain’s main car industry group, the Society of Motor Manufacturers and Traders, reiterated last week that a “no-deal scenario would have a devastating impact on investment and our hard-won reputation – risking the UK’s position as a leading global market and a centre of excellence for innovation”.

JLR’s factory shutdowns began as the carmaker, which is owned by the Indian conglomerate Tata, released full-year results. JLR sold 578,915 vehicles globally in the year to March, down 5.8%. In March alone, sales fell 8.2%, mainly because of an 11.4% decline at Land Rover, while Jaguar recorded a 0.2% dip.

The carmaker blamed weaker demand in China, whose economy has slowed sharply. JLR sales in China slumped 34%, while sales in Europe were down 4.5% because of uncertainty around the future of diesel vehicles, and the impact of new emissions legislation. The effects of the fuel efficiency and emissions testing procedure, called WLTP, have been felt across the industry.