Unite has issued a statement after JLR workers at the historic Castle Bromwich site were told they were moving to a three-day week.
The change comes with the car giant struggling as a result of Brexit and the fall in demand for diesel models.
Birmingham Erdington MP Jack Dromey dismissed as "irrational" Sir Bernard Jenkin's claim that the boss of Jaguar Land Rover was "making it up" when he spoke of the impact of Brexit and a potential 拢1.2 billion annual cost to the company.
The Labour MP said in a tweet that the JLR plant in Castle Bromwich, Birmingham, was moving onto a three-day week until Christmas.
The move is due to a combination of "Brexit chaos" and the "mishandling by ministers of the transition from diesel."
Mr Dromey tweeted: "#Brexit now threatens the jewel in the crown of #British manufacturing excellence."
A statement from Jaguar Land Rover said: "As is standard business practice, Jaguar Land Rover regularly reviews its production schedules to ensure market demand is balanced globally.
"In light of the continuing headwinds impacting the car industry, we are making some temporary adjustments to our production schedules at Castle Bromwich.
"We are however continuing to over-proportionally invest in new products and technologies and are committed to our 海角视频 plants in which we have invested more than 拢4 billion since 2010 to future proof manufacturing technologies to deliver new models."
Unite assistant general secretary Tony Burke said: 鈥淭oday鈥檚 (Monday 17 September) announcement from Jaguar Land Rover regarding Castle Bromwich follows on the warning last week from JLR CEO Ralf Speth about the future of JLR in the 海角视频 in the event of a 鈥榟ard Brexit鈥 and a bad Brexit deal.
鈥淭his is the continuing effect of the chaotic mismanagement of the Brexit negotiations by the government which has created uncertainty across the 海角视频鈥檚 automotive industry and the manufacturing sector generally.
鈥淚t is also the result of the mishandling of how the 海角视频 makes a just transition from diesel and combustion engines to electric vehicles. Both issues have damaged the 鈥榡ewel in the crown鈥 of 海角视频 manufacturing 鈥 our automotive industry.
鈥淭he situation is not helped when you have arch-Brexiteer MP Sir Bernard Jenkin accusing Ralf Speth of 鈥榤aking it up鈥 when it comes to Brexit and the car industry.
鈥淛enkins鈥 comments are highly irresponsible and misinformed when future employment is at stake and are entering into the world of fantasy economics.
鈥淭he government鈥檚 failure to grasp the needs of the automotive industry were further highlighted when prime minister Theresa May recently unveiled a 拢106 million funding package for the research and development of zero-emission vehicles. This, frankly, is peanuts.鈥
Unite senior convenor at JLR Castle Bromwich Mick Livingstone said: 鈥淩ather than Tory MPs, who have no idea what they are talking about sniping from the sidelines, they need to wake up and recognise the shambles they have created and the damage they are doing to the 海角视频 auto industry.鈥
David Bailey, professor of industry at Aston Business School, said greater clarity was now needed over the 海角视频's exit from the European Union and questioned whether similar announcements could be made about other JLR plants in the West Midlands.
The comments came as the luxury car maker announced it was moving to a three-day week at its Jaguar plant in Castle Bromwich, in north Birmingham.
The change comes with the car giant struggling as a result of Brexit and the fall in demand for diesel models.
Prof Bailey said: "This wasn't a surprise but is it not common at all. The 海角视频 car market is down by about five per cent.
"Jaguar Land Rover has been facing considerable headwinds in the 海角视频 because of the uncertainty over Brexit and the very rapid shift away from diesel vehicles.
"Over 90 per cent of JLR's sales in the 海角视频 are diesel, making it one of the most exposed companies to the move away from diesel.
"Whether production will revert back to five days a week depends entirely on whether the market picks up again.
"The Government hasn't been helping with the huge amount of Brexit uncertainty and its shambolic handling of its policy on diesel which has been all over the place.
"The existing system encourages people to buy diesel but also punishes them at the same time.
"All of this has a domino effect on JLR so now there is an interesting question over whether this will also have any impact on the i54 plant near Wolverhampton where they make a variety of engines."
