By Stephen Cape BBC News labour affairs correspondent

The GMB may call for the public to boycott Asda stores

Paul Kenny, acting leader of the GMB, claims Asda wishes to weaken the union and end its involvement in negotiations over pay and conditions.

The union has already balloted staff at 21 Asda depots over the dispute.

Asda, owned by US giant Wal-Mart, denies it is anti-union and has accused the GMB of having political aims.

Higher wages

In a letter to the TUC umbrella union group, Mr Kenny accuses Asda of offering staff higher wages in exchange for giving up their union membership.

He called on the TUC to back the GMB's campaign.

The GMB has 25,000 members among Asda's 150,000 UK workforce.

On 10 February an employment tribunal in Washington, Tyne and Wear, ruled that Asda had discriminated against trade union members.