Production of food institutions including Marmite and the Pot Noodle is under threat after three of Britain's largest trade unions voted for strike action at Unilever.

Unilever employees represented by Unite, the GMB and Usdaw are preparing for a series of strikes in a dispute over pension changes, starting with a 24-hour walkout. The consumer goods group has angered employees by closing its final salary pension scheme to existing members, and sources close to the discussions said walkouts are imminent unless a compromise can be reached.

"There is a real sense of betrayal over this," said Jennie Formby, Unite's national officer, who said the closure had come only three years after the scheme was shut to newcomers. "Significant numbers of workers are going to lose a major amount of what they expect to get when they retire."

Although scheme members will retain the benefits accrued so far, they will now move on to less generous pensions, Formby said.

According to Unilever, the final salary pension scheme has 5,000 active members and a deficit of £680m.

Formby said walkouts across Unilever's 12 UK sites are likely next month, hitting production of Marmite in Burton upon Trent, Pot Noodles in Crumlin and PG Tips in Manchester. Other products made by Unilever in the UK include Hellman's mayonnaise and, on the non-food side, Lynx, Sure, Persil and Comfort.

Unilever said in a statement that closing its final salary plan was the "right thing to do" to ensure it had a long-term future in the UK. "Our pensions arrangements in the UK have to reflect today's realities if they are going to be sustainable into the future and this means making tough choices, including an acceptance that final salary pensions are no longer a viable option for the company." Asked whether the looming strikes would affect production, a spokesman said: "We have had plenty of time to prepare for any industrial action – so our customers and consumers can rest assured that there will be no disruption to supply. "

About 2,500 of Unilever's 7,000 employees were balloted, with 85% of Unite members voting for walkouts. At Usdaw, the majority was 83%, while the GMB reported a majority of three to one. Allan Black, GMB's national officer, said "there is no doubt" strikes will go ahead. Unite and GMB members are also taking part in the national strike over public sector pensions on Wednesday.