The GMB union intends to hold a strike ballot among workers at 21 Asda distribution depots - an announcement that coincides with the Wal-Mart owned retailer's decision to appeal against an employment tribunal ruling that awarded workers at its Tyne and Wear depot GBP850,000 (US$1.5m) in compensation.

The employment tribunal found Asda to have breeched a law that prohibits employers offering employees incentives to relinquish collective bargaining rights.

According to the GMB, in the autumn of 2004 workers at the depot were offered a 10% payrise to give up union bargaining rights. The union says the supermarket operator's appeal is an attempt to justify its anti-union activities.

Asda spokesman Nick Agarwal told just-food that the chain denies any attempt to undermine the union, and it is because of this that the company has chosen to appeal against the employment tribunal's findings. "We disagree that there was an inducement to give up collective bargaining," he said.

The GMB said that the national strike ballot will be held in mid-April.