Yesterday, Northern Foods announced that it was closing its Trafford Park Bakery in Manchester with the loss of 690 jobs.
The site produces a range of quiches, sausage rolls and pork pies for Tesco and last year recorded pre-tax losses of more than £3m.
Jude Brimble, GMB national officer, said: "This is another example of Tesco throwing their buying power weight about by depressing prices and moving production at will to other plants.”
She added: “GMB will be asking the Office of Fair Trading to examine this specific case as an abuse of power.”
However, Pat O'Driscoll, chief executive of Northern Foods, said that the decision to close the factory was its own decision and not Tesco's.
A spokeswoman for Tesco added: “Over time we have been as supportive as we can be to improve the performance of the site. When Northern Foods told us that they were closing Trafford Park we asked if we could move this business to another site or sites but have been told there is not enough capacity to do this.”