The Competition and Markets Authority (CMA) regulator had been due to publish its provisional findings on the deal by early February and its final report in early March. The provisional findings will now come at some point in February, with a final report due on April 30.

The deal, agreed in April 2018, between second-ranked Sainsbury's and the British arm of Walmart, the number three player, could see the combined group vault Tesco as market leader.

The regulator said it needed more time to consider the deal due to the "scope and complexity" of the takeover and the need to consider the responses by the two main parties and other submissions.

The CMA has said its first priority was assessing if customers would face higher prices or a lower quality of service.

Tesco has said the deal should not be cleared without "extensive remedies", and number four Morrisons has raised concerns about an "effective duopoly" - Tesco and Sainsbury's-Asda - controlling more than 60 percent of the market.

Sainsbury's Chief Executive Mike Coupe has said that the company would challenge in the courts any unfavourable final ruling by the CMA on the deal if it believed it was not backed up by published evidence.

(Reporting by Kate Holton, editing by Louise Heavens)