By Emma Simpson
Business correspondent, BBC News
Asda's sale to forecourt tycoons the Issa brothers could raise petrol prices in some parts of the UK, the competition watchdog has warned.
The Competition and Markets Authority found "local competition concerns" regarding fuel in 37 areas in the UK.
Zuber and Mohsin Issa and TDR Capital agreed a £6.8bn deal last October to buy Asda from its US owner, Walmart.
The Issa brothers and TDR Capital also own EG Group, which operates 395 petrol stations in the UK. Asda owns 323.
The CMA launched an initial inquiry into the takeover in December last year.
The buyers now have five working days to address the competition concerns and avoid a more in-depth investigation.
Joel Bamford, CMA senior director of mergers, said: "Our job is to protect consumers by making sure there continues to be strong competition between petrol stations, which leads to lower prices at the pump.
"These are two key players in the market, and it's important that we thoroughly analyse the deal to make sure that people don't end up paying over the odds.
"Right now, we're concerned the merger could lead to higher prices for motorists in certain parts of the UK. However, if the companies can provide a clear-cut solution to address our concerns, we won't carry out an in-depth Phase 2 investigation."
A spokesperson for the Issa brothers and TDR Capital said: "We will be working constructively with the CMA over the course of the next 10 days in order to arrive at a satisfactory outcome for all parties within Phase 1.
"This would provide welcome certainty for our colleagues, suppliers and customers, and allow us to move forward with our exciting plans for investment and growth at Asda."