The Competition and Markets Authority (CMA) said it should not go ahead because it would raise prices for consumers.
The CMA said it wanted to “protect the millions of people who shop at Sainsbury’s and Asda every week”.
Sainsbury’s said it would not appeal against the decision.
The supermarket’s chief executive, Mike Coupe, said: “The CMA’s conclusion that we would increase prices post-merger ignores the dynamic and highly competitive nature of the UK grocery market.
“The CMA is today effectively taking £1 billion out of customers’ pockets.”
