Jaguar Land Rover workers observing Ramadan at the firm's factories in the region claim they were forced to break their religious fast because the temperature on the shopfloor was so hot.
Muslim workers attempting to adhere to strict Ramadan rules say they had to take on fluid during the day which contravenes the ritual.
Other staff members say the vehicle maker had failed to allow employees to open their food at certain times because of strict protocol over breaks.
One worker, based in JLR's plant in Castle Bromwich who has asked not to be named, said: "A Muslim DHL driver nearly passed out because of the heat inside and had to have a drink of water. He was really beating himself up over it.
"Muslims are not supposed to take fluids or take any food until it gets dark but it was so hot and humid in the factory."
A Solihull-based Land Rover worker said the temperature had reached 31 degrees Celsius in the Lode Lane factory at one stage during the recent record-breaking July heatwave.
"We work hard in a factory where the temperature is over 31 degress and we asked for fans. But in the area where I work there are no fans and we were told there is no budget for them," the employee said.
"There are people fasting and the company didn't even care when he asked for a fan to cool down because he can't drink water till night falls.
"The health and safety managers don't care as long as they are in their air-conditioned offices."
Another worker with Jaguar-based logistics firm Cat UK said: "Because of fasting, workers are taking one break instead of two but they have now been informed that they have to take two breaks.
"This can result in them having to wait until their shift ends to eat their food which can result in people passing out from hunger. What they are doing is wrong - they are not allowing them to follow their religious beliefs."
A spokesman for JLR said: "Jaguar Land Rover took proactive actions to ensure workers were made aware of the risks of the hot weather and steps they should take in the workplace during the hottest day of the year last Wednesday."