Tesco is facing a last minute challenge to its purchase of the London-based Europa, Harts and Cullens stores.
Tesco wants to increase the number of its smaller Express outlets
Trade body the Federation of Wholesale Distributors (FWD) has lodged an appeal with the Competition Appeals Tribunal to try to block the deal.
Tesco got clearance from the Office of Fair Trading on 5 March to buy the convenience stores from their parent company Adminstore.
But the FWD said the deal would be both bad for consumers and competition.
Tesco is buying the 45 stores for £53.7m as part of its policy of increasing the number of its smaller Express outlets.
Alan Toft, director general of FWD, which represents cash and carry and delivered wholesalers in the UK, said that if the deal went ahead "Tesco will virtually buy London with this acquisition".
Tesco has a 28% share of the total grocery market, but just 6% of the smaller convenience store market.