Food giant Kraft has been accused of breaking a promise to keep open a Cadbury factory after union officials said its closure was going ahead with the loss of up to 400 jobs.

The US firm said last week it would keep open the Somerdale plant in Somerset, which Cadbury’s had planned to close in the spring and switch production of chocolates to Poland.

Unite said it believed auditors were planning to announce moves to mothball the plant in the next few days, dashing workers’ hopes that their jobs could be saved.

“It looks like the first promise to be broken by Kraft,” said a Unite official. “We are calling on the government to intervene.”

Business Secretary Lord Mandelson met Kraft chief executive Irene Rosenfeld after the takeover was sealed last week but failed to win a specific commitment on jobs.

Cadbury announced in 2007 that it planned to close Somerdale and transfer the work done there to Poland with the loss of about 500 jobs.

About 100 workers have already been made redundant but Kraft’s ownership had offered hope that the remaining jobs could be saved.

Hundreds of Cadbury workers staged a noisy protest in Westminster last week to protest at the takeover and call on the Government to do more to protect their jobs.

No one was available for comment from Kraft.

The Somerdale plant is located in Keynsham near Bristol and Bath, and was originally built by the Fry family.

After the First World War, Cadbury Brothers merged with Fry & Sons, which was completed in 1919, leading to the relocation of production from sites in Bristol.

The transfer of production was completed in 1935, and at its height the Somerdale workforce was more than 5,000. It had its own power station and railway, with connection to the Great Western Railway via sidings at Keynsham railway station.

The factory was built with social facilities, including playing fields and a large recreational sports grounds, which still serve Keynsham.

Products manufactured at Somerdale include Fry’s Chocolate Cream, the Double Decker, Dairy Milk, Chocolate Buttons, Creme Eggs and Mini Eggs, Cadbury’s Fudge, Chomp and the Crunchie.

Unite urged Kraft to “come clean” on its plans for the entire Cadbury workforce, saying it had repeatedly questioned the firm’s commitments for the future of Somerdale.

Rumours were circulating that auditors were to begin moth-balling the plant barely a week after Cadbury changed hands, said the union.

National officer Jennie Formby said: “The ink is barely dry on the takeover and a promise made to the Cadbury workers has been broken. If the rumours now circulating are to be believed, Somerdale looks set to be the first casualty of Kraft’s ownership.

“Understandably, the Somerdale workers are starting to get very distressed by this mounting uncertainty. They have been on a rollercoaster of emotions since the shock of closure announcement over two years ago so when Kraft made repeated promises to save their plant they were thrilled. Now despondency and anger are setting in.

“Promises were made to the Somerdale workers which it seems Kraft had no intention of fulfilling, and which appear now to have been a cynical attempt to curry favour with the British public during what was an extremely unwelcome and unpopular takeover.

“This is not just a tragedy for this workforce but is also sending a very worrying message to the rest of the Cadbury workforce.”

Jack Dromey, Unite’s deputy general secretary, said that the Government must now step in to ensure Kraft ended the “chronic uncertainty” besetting the workforce.

“While Irene Rosenfeld may be talking to ministers and the media, no one is talking to Cadbury’s workers. Only last week, Lord Mandelson was pushing Kraft for assurances on jobs and investment but promises of a bright future are already being broken. The longer Kraft avoid dealing with the workers’ union, the more workers will suspect that cuts are coming. The Government should now act.”