Tesco has come under fire from local planning authorities for "riding roughshod" over the planning rules by making physical changes to its stores without consent.

Fresh accusations against the retailer over retrospective planning permission come the week before Competition Commission publishes the preliminary findings of its inquiry.

"Tesco knows if it goes through the normal planning process every time it wants to install new refrigeration plants or a new shop front, it may take up to four months," said a head of planning at one council in London. "If it has to wait four months to rip out the shop front and internal fixtures of a newly acquired independent , that represents four months' profit. It is worth more money to them to make the changes and worry about planning later."

The source said local authorities across the country had had similar experiences. "It often makes changes with total disregard for local residents," he said. "Tesco knows it isn't actually illegal to ride roughshod over the planning process."

A Tesco spokesman admitted it sometimes acted without full consent but claimed it was "for the good of consumers". "Our aim is to bring local stores to local people as quickly as possible," he said. "We always consult with planning officials so although it may not be with the consent of the planning committee, we are having conversations with local authorities. The process often takes years and as a business, you don't want to wait that long to install new environmental technology that can benefit customers and staff."

Councillor Robert Davis, cabinet member for planning at Westminster City Council, is campaigning to make it illegal to act without planning consent. "Where does it stop if you are breaking rules to benefit someone?" said Davis. "Other big retailers such as M&S and John Lewis find it easy to follow the rules."