Sheffield workers held a protest over plans to cut their overtime pay, Bank Holiday and weekend working rates.
Employees at Pennine Foods held a rally outside Marks & Spencer on Fargate in Sheffield city centre yesterday.
Union chiefs claim some workers could lose up to £5,000 a year from the changes.
Around 400 staff members - which makes M&S ready meals - plan further strikes.
Debra Loy, Bakers, Food and Allied Workers Union secretary said: “The owners want to cut our terms and conditions which means some of my weekend workers could lose up to £5,000 a year.
“Bank Holidays will be cut from time and a half and a lieu day to normal time and they’re just slashing everything to the bare minimum.
“Enough is enough, these people cannot come out of work with less wages.”
The union secretary added workers at the Waterthorpe site were being ‘punished’ for the introduction of the so-called National Living Wage which sees people aged 25 and over being paid £7.20 an hour.
Ms Loy added to balance this wage rise, weekend overtime and Bank Holiday rates would now become ‘normal time’.
“The morale is terrible on the shop floor because people are worried about the cut in their wages,” the union secretary added.
“We’ve carried out a two-day strike, we now given a letter in saying we’re planning another two-day strike and since the letter was given in at Monday morning at 9 o’clock, we’ve had no reply from the management team whatsoever so nobody has spoken to us at all.”
Pennine Foods worker Mark Cotton, aged 54, of Handsworth, has worked for the company since 2011.
He said: “There are married couple who work at the factory so for some, the household income could drop even more.
“It’s a drastic cut and it’s just not acceptable.”
Pennine Food’s parent company said it was ‘disappointed some colleagues have decided on industrial action’.
Over 84 per cent of members balloted for strike action.
A spokesman from Pennine Foods’ parent company 2 Sisters Food Group previously said: “We will be holding more talks to avert any further action, and have contingency plans in place to ensure production remains unaffected.”
The Star contacted for 2 Sisters again for a further comment but did not respond.