Staff at Ford's plant in Bridgend working on the new Jaguar engine have been told not to come into work for five days.
The AJ production line at the plant will shutdown from 29 October to 2 November, the GMB union confirmed.
Letters were sent to directly staff this week without informing union officials in advance.
Wales' first minister Carwyn Jones said he wanted assurances the move was only temporary.
Mr Jones, who is the AM for Bridgend, said on Twitter that he wanted to meet with Ford officials.
News of the shutdown has caused fresh anxiety for those at the plant - which is one of the biggest employers in Wales.
Last week, Ford's European boss Steven Armstrong warned a no-deal Brexit could affect the company's future in the UK.
And in 2017, it was announced production of engines for Jaguar Land Rover would end three months earlier than expected.
It is understood the workers affected will still get basic pay.
Analysis: Sarah Dickins, BBC Wales economics correspondent
The timing of the shutdown is somewhat ironic - as it comes into effect on the same day as the plant begins production of its new Dragon engine for Ford cars.
Investment in that new engine for Ford was welcomed, but there has been concern about the long-term viability of the plant.
The dragon programme will only be producing a quarter of the engines that the Bridgend plant has been making until now.
The latest move comes at a time when the automotive components sector starts to feel the impact of cuts in production by big car makers.
Jaguar Land Rover (JLR), owned by Tata Group, announced cuts in production at its plants in Solihull, Halewood and Castle Bromwich. As a result there is less demand for the parts that go into the cars.
The company has blamed falling demand in China and also "Dieselgate" - the UK government's change of policy on diesel cars and the subsequent fall in sales of diesel vehicles.
At the same time, car makers are becoming increasingly vocal about their fears about the possible effect of Brexit on the industry.
BMW said in September it would stop producing its minis at Cowley in Oxford for a month when the UK leaves the EU.