Confectionery giant Nestle is planning to cut almost 300 jobs and move production of the Blue Riband chocolate biscuit to Poland, unions said.
The job losses will mainly affect the company's sites in York and Newcastle as well as a small number of cuts in Halifax, West Yorkshire, and Girvan, Ayrshire.
The GMB and Unite unions both criticised the news, with the GMB saying Nestle should "be exporting chocolate and not jobs".
The company is proposing 143 job cuts in York, 110 at Fawdon, 15 in Halifax and seven in Girvan, the unions said.
GMB general secretary Tim Roache said: "To shift the production of an iconic British brand like Blue Riband to Poland is completely unacceptable.
"Nestle are throwing people's lives, and those of their families, into turmoil for the sake of increasing profit margins.
"These factories should be exporting chocolate - not people's jobs.
"The Government needs to step in before it's too late and reassure millions of workers across the country this is not just the tip of the Brexit iceberg."
Nestle said in a statement: "These proposals span four different sites: York, Fawdon, Halifax and Girvan and may result in a reduction of 298 roles, predominantly at York and Fawdon, through 2017 and 2018. It is expected that these would be achieved through voluntary redundancies.
"The proposed changes include amended and standardised shift patterns at each factory and, at Fawdon, the most complex of Nestle's UK confectionery sites, the transfer of Blue Riband production to a Nestle factory in Poland. This would mean being able to simplify and focus Fawdon's operation.
"These proposals are being made by Nestle UK to ensure that these sites operate more efficiently and remain competitive in a rapidly changing external environment.
"A 45 day consultation on these proposals will commence as soon as possible with trade unions and employee representatives. Nestle UK appreciates that this is an uncertain time for employees and will work hard to ensure all are supported through this difficult period."
Unite national officer Julia Long said: "We will be campaigning to save as many jobs as possible and pressing Nestle to think again about these plans.
"Rather than turning its back on its UK workforce, Nestle should be investing in its UK operations and keeping production here at plants in the UK.
"Over the coming days we will be scrutinising the company's business rationale for these job losses and explore alternatives to its cut-and-run approach."
Valerie Scott, a GMB northern region officer, said: "Pillaging our jobs and shipping them overseas looks like the worst kind of Brexit imaginable.
"Under this Government it's more the Northern Poorhouse than Northern Powerhouse."