Hundreds of workers at one of Norfolk's biggest factories are facing a pay cut and changes to their shift patterns as their employer struggles to maintain profit margins.

Bowthorpe-based Kettle Foods says the changes will help plug a profit gap left by a price war with discount retailers – but some workers have claimed the changes could leave them thousands of pounds worse off every year.

In a letter to workers, the company said competition from discount retailers such as Aldi and Lidl had forced it to bring down product prices faster than it could reduce costs, meaning profits had halved since 2014.

The proposed changes mean employees will work up to 151 shift of 12 hours a year under a new pay structure. But staff unhappy with the proposals claim 'nearly every employee' would see a reduction in salary due to the new grading structure, with most production workers losing 'between £3,000 and £6,000 a year'.

One employee, who wished to remain anonymous, said he stood to lose as much as £6,400 from his salary. 'The managers tell us we are doing really well and the next thing we know we get an email saying we are getting a pay cut, so as you can imagine there are a lot of unhappy bunnies,' he said.

In its letter Kettle said no changes would be implemented until July 31 2018, and that staff facing a pay cut would have their wages gradually reduced over a six-month period. Some 470 staff are employed at the factory, though the number affected has not been confirmed.

Annual holidays will also be affected, with staff only able to take holiday during factory shutdown periods or on days when they are not rostered to work.

Some non-UK staff worry this new requirement means they will not be able to book enough time off to visit their families abroad.

Kettle refused to comment on the employees' claims as a consultation on the proposals is ongoing, but in a statement it said: 'As we head into 2018, we are pleased to report that Kettle Foods will be increasing production at the Norwich factory after having secured additional business with several key customers.

'As a result, we are reviewing the infrastructure of our Norwich site to ensure we are able to meet the growing demands on the business.'

The firm added that this is 'not a redundancy situation', but it 'believes' it will find roles for all existing permanent employees.

Kettle Foods is a member of the EDP/EADT Top 100 list of Norfolk and Suffolk's biggest companies by turnover.

The company is owned by Snyder's-Lance and turned over £120.37m in the 17 months to December 31 2016 with pre-tax profit of £8.25m.