The Competition and Markets Authority has launched the first phase of its investigation into the mega-merger between the two members of the big four.

The CMA has launched a preliminary invitation for stakeholders to comment on the Sainsbury’s-Asda merger, first revealed last month.

The proposed merger is a landmark deal that will create a supermarket powerhouse with more than 2,800 stores.

The businesses hope to create a new combined and enlarged business that will be a “dynamic new player in UK retail with an outstanding breadth of products, delivered through multiple channels”.

The merger comes after Tesco’s acquisition of convenience player Booker was passed by the CMA.

Much of the market was surprised that the authority had chosen to pass the merger. Former Asda boss Andy Clarke went so far as to say the CMA’s stance on the acquisition had declared “open season” on retail M&A activity.