The move would be a severe blow to Morrisons, which is struggling with the integration of the Safeway chain which it bought for £3.3bn last year. If drivers, warehouse and support staff walk out, supermarket shelves could be empty within a week. GMB national officer Jude Brimble said: 'What we don't know is which sites will close and we want to know what's happening with members' jobs. We think the old Safeway depots like the one in Warrington are more vulnerable.' A Morrisons spokesman said it was 'no secret' it was conducting a review of its distribution network. He added: 'Regrettably the GMB has consistently refused our offers to talk about how the integration process of Safeway and Morrisons might affect their members. Our door remains open.'