The U.K.'s Competition and Markets Authority delayed its plan to hand down initial results of its review of J Sainsbury PLC and Walmart Inc.-owned Asda Stores Ltd.'s proposed merger.

The regulator previously said it expects to issue its provisional findings "early next year," but in the updated timetable released Dec. 13, it said it will release the findings in January or "early February."

The CMA still expects to publish the final report in early March.

The move came shortly after Sainsbury's and Asda sought an extension into the CMA's review "given the unprecedented scale and complexity of the case."

In response, the regulator said that if it agrees to give the companies more time, it would put its ability to complete the review by the required deadline "at very serious risk."

It added: "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."

The CMA said the size of Sainsbury's and Asda's merger requires assessing a large volume of evidence in a short timeframe. It has sought statements from other players in the market, including Wm Morrison Supermarkets PLC, Tesco PLC, Waitrose Ltd., Lidl Stiftung & Co. KG, Ocado Group PLC and Amazon.com Inc., which operates grocery delivery service AmazonFresh in the U.K.