SUPERMARKET group Asda is threatening to use legal action to block a potential strike over pay.

Workers in Asda depots across the UK - including Washington and Teesside - are being balloted on strike action over a lengthy collective pay bargaining row.

If workers back a strike, the GMB union would need to give at least seven days' warning of industrial action.

The supermarket's chief executive met GMB union officials at their annual congress in Blackpool last week but neither side could make progress.

The result of the strike ballot is expected to be announced tomorrow afternoon.

Union officials have expressed concern that Asda is drafting in agency workers to help deal with the prospect of depleted staff numbers should any strike action go ahead.

But a company spokeswoman said: "There a very clear rules in place for employing agency labour during industrial action - if it comes to that - and we have absolutely no intention of breaking them."

Asda has written to the GMB threatening legal action, particularly as it believes the union has overestimated the number of Asda's 12,500 depot workers belonging to the union at its 24 depots around the country. The union believes the figure is about 7,000 while Asda says it is closer to 4,500.