Prices could rise at a large number of Sainsbury’s and Asda petrol stations if the proposed merger of the two companies goes ahead, the Competition and Markets Authority (CMA) has warned.

In its provisional findings on the deal its in-depth investigation of the proposed merger found extensive competition concerns.

Stuart McIntosh, chair of the independent inquiry group carrying out the investigation, said: “These are two of the biggest supermarkets in the UK, with millions of people purchasing their products and services every day. We have provisionally found that, should the two merge, shoppers could face higher prices, reduced quality and choice, and a poorer overall shopping experience across the UK. We also have concerns that prices could rise at a large number of their petrol stations.

“These are our provisional findings, however, and the companies and others now have the opportunity to respond to the analysis we’ve set out today. It’s our responsibility to carry out a thorough assessment of the deal to make sure that the sector remains competitive and shoppers don’t lose out.”

In its findings the CMA said it believes the deal could lead to inflated fuel costs at more than 100 locations where Sainsbury’s and Asda petrol stations overlap.

A spokesperson for Sainsbury’s and Asda responded: “These findings fundamentally misunderstand how people shop in the UK today and the intensity of competition in the grocery market. The CMA has moved the goalposts and its analysis is inconsistent with comparable cases.

“Combining Sainsbury’s and Asda would create significant cost savings, which would allow us to lower prices. Despite the savings being independently reviewed by two separate industry specialists, the CMA has chosen to discount them as benefits.

“We are surprised that the CMA would choose to reject the opportunity to put money directly into customers’ pockets, particularly at this time of economic uncertainty.

“We will be working to understand the rationale behind these findings and will continue to press our case in the coming weeks.”

The CMA has set out potential options for addressing its provisional concerns. These include blocking the deal or requiring the merging companies to sell off a significant number of stores and other assets – potentially including one of the Sainsbury’s or Asda brands – to recreate the competitive rivalry lost through the merger. The CMA’s current view is that it is likely to be difficult for the companies to address the concerns it has identified.

The CMA will take responses from interested parties to its provisional findings by 13 March, and its notice of possible remedies by 6 March. Its final report is due to be issued by 30 April.