The Unite union said the local shift pattern has been agreed between Unite and JLR on behalf of a small group of workers.
It does not affect workers in wider production roles at the centre on the i54 business park at Wolverhampton.
The shifts involve a four days on, four off pattern.
JLR said the shift pattern and hours involved were as per existing employment terms and conditions and in agreement with the trade union.
“This change is in response to the growing demand for the engines manufactured at the Wolverhampton Engine plant. The weekly hours will increase from 39 hours to 42 hours a week and includes a shift premium and overtime rate to reflect the shift and additional hours worked.
“Unite will endeavour to continue to harmonise terms and conditions across all JLR sites through negotiations on a national level," said the union's senior communications officer Barckley Sumner.
An EMC worker, who did not wish to be named, claimed that a sub-set of workers are planning a strike in response to the 42 hour working week, four on, four off shift pattern.
He said the union had not consulted the workers or balloted them.
The worker also claimed: "A lot are threatening to leave Unite. We are paid a lot less than Solihull or Castle Bromwich and we workers want the public to know this. Those sites do 36 hours and we do 39 now, but will be doing 42. We just want standardisation with the other JLR sites.
"We love working for the company but just want some recognition and consultation from the management and union for spending even more time away from our families."