Gunstones Bakery, based in Dronfield, near Sheffield, lost the contract for chilled foods in June, but the full impact is only emerging now: more than 500 jobs are at risk.
Union officials said up to 524 posts were affected, which represents 45% of the site workforce, according to bbc.com.
The company said it was working to transfer workers and a new separate contract would help with this.
Organizing officer for the Bakers, Food and Allied Workers Trade Union (BFAWTU), Jit Singh, said: “We have already held talks with the company and these have been very constructive. While it is too early to know what the full impact will be, there are opportunities at other sites, some of them quite local.”
A spokesman for 2 Sisters Food Group, which owns the bakery, said: “We announced to colleagues in June the loss of a filled rolls and sushi contract to another supplier. As this contract doesn’t end for another six months, this gives us time to put plans in place to mitigate this loss, and this continues. In addition, we have won a new five-year bakery contract since we announced the contract termination.”