Hundreds of people faced an uncertain future at Dairy Crest on Tuesday after the milk supplier said it was looking at closing two facilities in Liverpool and Cambridgeshire.
The firm said it plans to close its factories in Aintree, which employs 220 people, and Fenstanton, where 250 people work.
The company claimed it was battling an "extremely challenging" market for liquid milk and the plans would help it reduce costs and sustain profitability.
Dairy Crest said the proposed closures had been facilitated by a recent £75 million investment programme in its dairies at its other three polybottle dairies at Severnside, Gloucestershire, Chadwell Heath in London and Foston, Derbyshire.
The firm - known for its butter and spread brands Country Life, St Hubert Omega 3 and Clover as well as Cathedral City cheese and flavoured-milk drink Frijj - said Tesco had also decided not to renew its contract with the company after its expiry in July.
Tesco declined to specify the reasons behind its decision, but said there would not be a tender process to replace Dairy Crest with another supplier.
A consultation has now begun into the job cuts with the Unite union.
National officer Cath Speight, said: "This is devastating news for the 400 workers spread across 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. As more and more people are made redundant, job opportunities are becoming even scarcer.
"We now owe it to our members and their families to do everything in our power to help and support them.
"We hope Dairy Crest's management will rethink this decision and we urge them to offer the staff affected every opportunity for redeployment within the business."
Dairy Crest chief executive Mark Allen claimed 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," he said.
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