The GMB has warned that Asda could be breaking the law if it brings in extra staff as strike cover to ensure stores don't run short during the World Cup.
Asda and the GMB had been negotiating over working conditions, bonus payments and union recognition before discussions broke down and the GMB called a strike ballot last month. The ballot closes on 21 June.
Asda had said it would be "working overtime to make sure our customers don't miss a thing when they visit our stores" in the event that a strike took place (The Grocer, 13 May, p9).
But a GMB spokesman told The Grocer: "Asda is attempting to drive a coach and horses through the Employment Agencies Act 2003, which prohibits agencies from supplying staff to a company in dispute."
But an Adsa spokesman said: "This is a relatively new piece of legislation but Asda is well aware of the law and we certainly won't be breaking it."