More than 2,000 staff at 17 Marks and Spencer stores across the country are going on strike today.
UPDATE: The chain has opened some of its smaller stores around the country by using temporary seasonal staff.
The row is over a decision by management at the company to close their workers defined pension benefit scheme - even though it is thought to be performing.
According to the trade union Mandate, the move came as a shock.
Cillian Doyle from People Before Profit said that their decision has left workers with no choice.
"I think the unions are working under the assumption that there would be talks," he said.
"I know, from talking to people in Mandate that they thought previously that they had a good relationship with the management, and they're actually quite shocked - I talked to one or two people in the Mandate union - and they're actually very surprised and it kind of took them by surprise.
"But they're very resolute now and they're determined to go on strike and to fight this."
Marks and Spencer have released a statement claiming that the unions are misleading their staff and their customers.
The store claims only a third of employees are affected by the change in pension schemes and not the majority, as the unions claim.
The company also said that it wants to re-open "meaningful negotiations".