Three weeks of strike action among Argos staff is on the cards in an ongoing dispute over redundancy payments.

The Unite union said the retailer had failed to negotiate a national agreement covering redundancy and severance packages and had failed to guarantee workers’ future terms and conditions after 500 staff transferred from its Lutterworth hub to Wincanton Logistics in Kettering.

Argos said the union’s demands were unreasonable and wholly unnecessary.

Unite warned deliveries to customers would be “severely hit” when the strikes start on 15 August and end early in the morning of 5 September.

The sites affected are Basildon in Essex, Bridgwater in Somerset, Castleford (West Yorkshire) and Heywood (Greater Manchester), which are all covered by the employer/union national forum, with workers at the Burton-on-Trent site, who wish to be covered by the forum, joining them.

The union said it was worried about the intentions of Argos, now owned by supermarket giant Sainsbury’s.

“We are faced with the thin end of the wedge with Sainbury’s pulling the strings behind the scenes – and the not-so-hidden agenda is serious cost-cutting to the detriment of our members,” said Matt Draper, Unite national officer for logistics and retail distribution.

“The transfer of the workers from Lutterworth to the Wincanton site at Kettering, whether they wanted to go or not, led to this strike ballot.”

An Argos spokeswoman said the dispute was not about pay, benefits or job losses, but was based on “entirely theoretically scenarios”.

She added: “We’re disappointed with the union’s actions, which are based on unreasonable demands and are wholly unnecessary. Customers will not be impacted and should be reassured that full contingency plans are in place.”

Argos said it was working with Unite and had made several proposals to satisfy the union’s objectives.