There are mounting concerns that the Competition Commission could be heading for a clash with the government over out-of-town planning, following the introduction of the Sustainable Communities Act last month.

In its preliminary findings, the commission recommended the relaxation of planning restrictions on edge-of-town sites, arguing that they acted as a barrier to entry. However, the government's Sustainable Communities Act, which gained Royal Assent last month, has given local authorities the power to block such developments to protect independent shops and post offices.

A "definite clash" was looming, said Ken Parsons, CEO of the Rural Shops Alliance. "The commission said local authorities were blocking supermarkets from opening new stores, but authorities have now been given more power to do this. It's a case of the left hand of the government failing to see what its right hand is doing," said Parsons.

The government was unlikely to introduce measures that conflicted with those in the new Act, he warned, adding that if they wanted to, local authorities could now stop supermarkets from gaining planning consent for out-of-town sites. "The Sustainable Communities Act could turn out to be one of the most powerful pieces of legislation ever passed by government. We will be contacting councils to alert them to the possibilities the Act could bring."

There was "tension" between the two approaches, said one leading competition lawyer. "The Sustainable Communities Act favours proposals that would contribute to sustainable local communities. The proposals most likely to be favoured would be those that foster viable and thriving town centres over out-of-centre retail sites," he said.

Nick Hurd, Conservative MP for Ruislip-Northwood, who was responsible for tabling the Act, said the commission's proposals went "against the grain" of efforts to get rid of so-called Ghost Town Britain.