The GMB has written to recruiters that supply Asda distribution centres with temporary staff reminding them it is against the law to do so if it breaks the strike.

The union has written to 62 agencies across the UK which supply staff to Asda’s 21 depots.

However the retailer says it has no intention of using temps as cover. It said: “The union is trying to confuse the issue. The fact of the matter is that we use agency staff every single day of the year at our depots, to cover everything from staff leave to big surges in demand such as the World Cup.”

Asda employees are unhappy at not receiving their annual bonuses because the retailer failed to meet its targeted annual profit, and because of a bid to increase their work rates.