The union Usdaw said hundreds of workers at the site in Aintree were set to strike from 0500 BST on Sunday. It said staff were unhappy after being offered what it said was a below-inflation pay rise of 3%.

A spokesman for Ladbrokes, which also has a call centre in London, said, "It is essential that our costs remain competitive. I think we have made a fair offer but we remain open to negotiations from the union."

It is the second time staff have gone on strike over the matter - they also walked out in August.

Usdaw's area organiser Michelle Owens said: "Staff on site are furious the company has not increased its 3% pay offer at a time when inflation is nearer 5%.

"Despite meetings with senior management and Acas the company have refused to improve the offer.

"Our members are struggling to cope with higher fuel and food bills and deserve better from the company."

She added: "While we are keen to resolve this dispute, everything hinges on the company's willingness to make an improved offer."