Thousands of workers at Asda are to stage a five-day strike over pay and bargaining.
Members of the GMB union at 20 distribution depots will walk out from June 30.
The strike will be followed by a "comprehensive programme" of industrial action.
The strike dates were agreed yesterday by GMB shop stewards following a 3-1 vote in favour of stoppages.
Drivers and warehouse workers will join the walk-out at depots across the country including Bedford, Chepstow, Dart-ford, Didcot, Falkirk, Grangemouth, Wigan, Lutterworth in Leicestershire, Bristol, Skelmersdale and Wakefield.
The union said it was aware that Asda, which is owned by American firm Wal-Mart, was gearing up to try to break the strike.