Premier Foods, the firm behind Branston Pickle and Oxo stock cubes, has announced plans to shut six factories, putting more than 1,000 jobs at risk.
The sites earmarked for closure include a jam factory in Ledbury, Herefordshire and others in Bristol, Reading and Manchester. These will shut by 2009.
Many belong to Hovis-owner RHM, which Premier recently bought for £1.2bn.
The company said it planned to offset about half of the job losses by creating positions at other sites.
Jobs would be created at the firm's remaining five sites, it stressed, which will all receive significant investment.
These include plants at Histon, Cambridgeshire, which will become the focus of preserves production, Worksop in Nottinghamshire, Knighton in Staffordshire and Ashford in Kent.
"These proposals will enable us to enhance our efficiency whilst giving us the capability to drive our branded sales growth," said Robert Schofield, chief executive of Premier.
"We appreciate the impact our proposals will have on employees at those sites and we will work with them to explore opportunities for redeployment within the group or outside of the company," he added.
Brian Revell, national officer of the union Unite, said: "The consolidation within Premier Foods will cause great hardship to over 1,000 workers and their families.
"We will fully support our members in opposing these proposed closures. Premier Foods is acting like a private equity company, without regard for the workers who have built these businesses over many decades."