Dairy Crest jobs to go in Liverpool and Cambridgeshire
Up to 470 people could be made redundant from Dairy Crest plants in Liverpool and Cambridgeshire.
The milk supplier has announced proposals to close its factories in Aintree, which employs 220 people, and Fenstanton where 250 people work.
The company said it was battling an "extremely challenging" milk market and the changes would help it reduce costs and sustain profitability.
A consultation has begun into the proposals, planned for later this year.
Chief executive Mark Allen said the closures would protect the future of the dairies side of the business.
"The decision to consult on the closure of our Aintree and Fenstanton facilities has not been taken lightly, but we believe that this proposed restructuring of our dairies business is the right decision for the long term."
Dairy Crest said the proposed closures had been facilitated by a recent £75m investment programme.
It said the group's other three "polybottle" dairies, at Severnside, Gloucestershire; Chadwell Heath, north-east London; and Foston, Derbyshire, were now more efficient and had increased capacity.
Toby Brinsmead, managing director of Dairy Crest liquid products, said: "We will do all we can to find [staff] alternative work outside or within Dairy Crest, if we possibly can... but I appreciate for the vast majority we'll be trying to help them find alternative work.
"This is just a proposal at the moment, but should the decision go through we are likely to close the dairies towards the end of 2012."
Cath Speight, national officer at the Unite union, said it was "devastating news" for staff at both sites.
"We have now entered into the 90-day consultation and Unite will be doing everything possible to save these jobs, protect our members' interests and maximise the opportunity for redeployment.
"These closures will be a massive blow for the workers and the local areas affected.
"We hope Dairy Crest's management will rethink this decision."