Drivers delivering for Land Rover are apparently being told they will get the sack if they don't quit the union.
A story on BirminghamMail.net claims drivers working out of Lode Lane could strike over the decision disrupting the supply chain.
The website reports 280 drivers, employed by logistics firm Autologic, were sent a letter by their employer saying: "Autologic can offer you good pay, good conditions and a good future. The trade union can offer you tea and biscuits when you lose your job.
"You must decide what it is you want. We will not be able to offer job security and good futures unless existing working practices are changed and the trade union stranglehold on this company is removed."
Ron Webb, the UK's most senior officer in the logistics sector for Unite said: "This employer, with a new chief executive and new human resources director, has unleashed an attack on the trade union, the like of which I have never seen in 30 odd years with this union."
A Land Rover spokesman said at this stage it couldn't comment.