The two supermarkets are to apply for a judicial review of the Competition and Markets Authority (CMA) investigation into their proposed tie-up, as they request more time to consider the evidence.
But the watchdog has said it will defend its position in court, and is “not willing to compromise on the thoroughness or objectivity” of the investigation.
The application, lodged on Wednesday with the Competition Appeal Tribunal, will request a review of the CMA’s timetable and process.
Sainsbury’s and Asda said they have asked the CMA for an additional 11 working days over the Christmas period to respond to a large amount of material which has recently been provided to them.
Both retailers argue the current timetable does not give them or the CMA “sufficient time to provide and consider all the evidence, given the unprecedented scale and complexity of the case”.
In a joint statement, Sainsbury’s and Asda said: “This is a case of unprecedented size and complexity and we have a responsibility to our customers and colleagues to ensure that we and the CMA have enough time to make and consider all the facts and evidence.
“This is not a decision we have taken lightly. It is about ensuring a thorough process and reasonable timetable. We remain confident in the case for merging the businesses and the significant customer benefits.”
However the CMA said the timeline is typical for an investigation.
A spokesman said: “As you would expect, investigating any merger of this size requires assessing a large volume of material in a short time-frame, 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.
“We are not willing to compromise on the thoroughness or objectivity of our investigation and will robustly defend this case in court.”