General trade union GMB has spoken out after US company Mondelez International acquired the licence to make Cadbury's biscuits from Burton's Biscuits.
GMB members working for big brands such as Maryland cookies, Viscount, Royal Edinburgh Shortbread and Cadbury Fingers are now concerned for their jobs if biscuit making is taken overseas.
Mondelez, previously Kraft, closed Cadbury's Somerdale in 2010 and moved Dairy Milk production to Poland.
The company has also been under fire for changing the Crème Egg recipe and widening the gap between triangles of a Toblerone bar.
During her takeover in July, Prime Minister Theresa May promised any takeovers of UK businesses by foreign companies would only be given the nod if they benefit British industry. She said: "a proper industrial strategy wouldn't automatically stop the sale of British firms to foreign ones, but it should be capable of stepping in to defend a sector".
Eamon O'Hearn, GMB national officer, said: "Staff at Burton's work extremely hard to make some the UK's best-loved biscuits and our members deserve to know their jobs are safe. Mondelez has a history of broken promises - and we have no idea what their commitment is to production in the UK. They need to come out and tell us exactly what their plans are for our members working for Burton's making Cadbury's biscuits. And Theresa May must stand by her pledge to protect British businesses from foreign takeovers."
Bill Esterson, shadow minister for business, energy and industrial strategy, said: "It is remarkable that the government says the takeover is a commercial matter when it will affect thousands of British jobs and the wider economy. The government claims it supports an industrial strategy, which should mean being partners with business and the trade unions. The Secretary of State should meet the GMB and listen to their concerns, and he should meet Mondelez. Foreign investors are vital to the success of our economy but they should not be breaking promises made to the British people. The government has a duty to safeguard the interests of the British workforce."