The two retailers are requesting a judicial review into the timetable and process, saying the “current timetable does not give the parties or the CMA sufficient time to provide and consider all the evidence given the unprecedented scale and complexity of the case”.

The competition watchdog referred the proposed tie-up for a more in-depth review in September after identifying “sufficient concerns” during its initial investigation.

A statement on Sainsbury’s website said: “Both parties have engaged constructively with the CMA to date and have made repeated requests for additional time. Specifically, we have asked the CMA for an additional 11 working days over the Christmas period to respond to a large amount of material recently provided to us.”

The CMA said giving the companies extra time now would put their ability to complete the investigation by the required March deadline “at very serious risk”.

In a statement, the CMA said: “Our first priority in this investigation has, and will continue to be, assessing if shoppers would face higher prices or a lower quality of service as a result of the merger and, if so, to prevent that from happening.

“We began to engage with the companies involved as soon as they announced their intention to merge at the end of April, to make sure they had the opportunity to fully put forward their views. Investigating any merger of this size requires assessing a large volume of material in a short timeframe, and it is not unusual for the companies involved to do this in the timelines we have been working to with Sainsbury’s and Asda. We have done everything we can to aid their consideration of this work, whilst still ensuring we are able to meet our legally-binding deadline. This includes extending certain administration timelines where appropriate.

“If we gave the companies the extra time they are now asking for, it would put our ability to complete the investigation by the required deadline at very serious risk. As with all of our merger reviews, we construct our timetable to ensure that everyone has the chance to have their say, including customers, the companies involved and suppliers.”

Sainsbury’s added: “We are confident in the merits of the deal and our ability to deliver the synergies. By bringing our two businesses together, we will invest further in range, quality and customer service, while lowering prices and reducing the cost of living for millions of UK households.”

A merger between the two supermarket giants was first announced in April where the brands said they expected their stronger purchasing power would deliver lower prices – up to 10% on many frequently bought products.