The Competition and Markets Authority (CMA) said it will look at whether the multi-billion pound deal could lead to less choice, higher prices and worsening quality of service for consumers if it gets the go ahead.

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.

But the CMA said it will look at the merger, including in-store, online, fuel, electrical, toys and clothing to see if the increased buying power would actually squeeze suppliers, and ultimately shoppers.

Andrea Coscelli, chief executive of the CMA, said: “About £190bn is spent each year on food and groceries in the UK so it’s vital to find out if the millions of people who shop in supermarkets could lose out as a result of this deal.

“We will carry out a thorough investigation to find out if this merger could lead to higher prices or a worse quality of service for shoppers and will not allow it to go ahead unless any concerns we find are fully dealt with.”

The CMA is inviting views on the merger until 31 August but a date for its decision has yet to be published.