Britain's competition regulator has blocked Sainsbury's proposed 7.3 billion pound ($9.4 billion) takeover of Walmart owned Asda — a huge blow to the supermarket groups who wanted to combine to overtake market leader Tesco.

The Competition and Markets Authority (CMA) said in its final report on Thursday that the deal would lead to increased prices in stores, online and at many petrol stations across the U.K.

Sainsbury's, Walmart and Asda said they had mutually agreed to terminate the transaction.

Sainsbury's and Asda agreed the deal in April last year. As well as overtaking market leader Tesco, it also would have given Walmart a way to exit Britain, one of the weakest performers in its global portfolio.

However in its final report the CMA found that UK shoppers and motorists would be worse off if Sainsbury's and Asda merged.

It concluded that the deal would result in a substantial lessening of competition at both a national and local level.

It said this would mean shoppers right across the UK would be affected, not just in the areas where Sainsbury's and Asda stores overlapped.

"We have concluded that there is no effective way of addressing our concerns, other than to block the merger," said Stuart McIntosh, chair of the CMA inquiry group.