Shares in Sainsbury's have fallen 5% after the retailer said it was seeking to delay an investigation into its proposed merger with Asda.
The Competition and Markets Authority (CMA) has denied its request for an 11-day extension but Sainsbury's said it would appeal against the decision.
The move raised fears the £15bn tie-up could be rejected when the regulator gives its final ruling next year.
It is probing whether the deal could harm competition and push up prices.
Sainsbury's takeover of Asda would create Britain's largest supermarket group, overtaking Tesco.