The supermarket group has been in a long-running dispute with the GMB union over working conditions and union recognition.
However, Asda is now claiming that the GMB may be overestimating its membership. The GMB claims that 7,000 Asda workers belong to the GMB, while Asda estimates that the number is nearer 4,500.
A spokesman for Asda said: “Unless we get reassurances from the GMB that the number of ballot papers issued does match the number of people who work for Asda, legal action is something we would consider.”
The union's strike ballot closes on Wednesday (21 June).