Weetabix staff at two factories are set to walk out over plans to 'fire and rehire' them on deals that would leave them £5,000 worse off a year.
The move will mean shortages of Weetabix and other products made at the factories, including Alpen, Weetos and Oatibix.
Unite, the union, said a series of 48-hour strikes will start on Tuesday, September 2.
They will then continue on every Tuesday until November 30.
The workers are based at the Weetabix factories in Kettering and Corby. Unite said they face changes to their shift and working patterns which would result in some being up to £5,000 a year worse off.
Fire and rehire, as the name suggests, is the employment of laying off workers and then offering them their jobs back with reduced pay or worse conditions.
Are you a Weetabix worker affected by this? Get in touch: mirror.money.saving@mirror.co.uk
Unite general secretary Sharon Graham said: “The idea of ‘fire and rehire’ is abhorrent to me.
"If Weetabix decide to go down this route and they overstep the line then I will absolutely defend our members.”
Strike action was originally scheduled for June but was postponed to allow for more talks with Weetabix.
Those talks did lead to new conditions being drawn up, but these were rejected by 82% of workers.
A Unite statement said consumers will "lose their appetites for a product produced by a highly profitable company like Weetabix which then attacks workers' wages".
Unite regional officer Sean Kettle said: “Strike action will inevitably lead to a disruption in production and shortages of Weetabix and the company’s other products will quickly follow.
“Weetabix is the UK’s favourite cereal but consumers are bound to quickly lose their appetite for the product when they learn it is made by mistreated workers."
Strikes could still be avoided, but Kettle said this would need Weetabix to drop plans to fire and rehire and then come up with a "realistic" offer.
A spokesperson for the Weetabix Food Company said: "To continue meeting the expectations of our customers and consumers, it’s important that our ways of working evolve. As a business, we continue to invest in our people, plants, and products.
“We’re naturally disappointed by the result of the Unite ballot, but respect the voice of our workforce and their representatives."
The spokesperson said it would keep talking to its workforce and that any shortages of the company's foods should be avoided.
They added: "It is unfair and inaccurate to compare this with other disputes that require new contracts to be signed or face dismissal, this is not a choice we're considering at present."
Weetabix, founded in 1932, was bought by US firm Post Holdings in 2017 for £1.4billion.
A report earlier this year into fire and rehire found its use was “widespread” by employers.
The Advisory Conciliation and Arbitration Service (Acas) was warned its use could increase as the government’s furlough scheme and business support measures end.
