It has quizzed retailers, wholesalers and suppliers. Now it's planners' turn.

This week the Competition Commission fired off questionnaires to every planning authority in England, Wales and Scotland as part of its investigation into UK grocery.

It hopes the responses, due in by Christmas, will help the team get a clearer picture of how planning authorities decide whether or not to allow retail development, and whether murky practices go on.

Planning is one of the key strands of the inquiry, with all major retailers griping about the existing system in their own submissions. Criticisms include claims the system is too slow and indiscriminate.

Opponents of big retailers argue supermarkets exploit the system by railroading authorities into approving stores and amassing landbanks to prevent rivals building stores near their own.

The questionnaire contains 22 questions - most of them multiple choice.

They include whether the cost of fighting an appeal deters the authority from standing up to a retailer whose application it has rejected. Some claim the high price to an authority of an appeal works to the benefit of rich retailers.

Other questions include whether the authority considers the number of alternative grocery retailers that operate within a locality.

Critics of the planning system say it does not facilitate consumer choice because it does not recognise store fascias when considering a plan.

It also asks if the authority has evaluated whether there are sites in its jurisdiction that could support grocery retail sites of over 280m2 in size, and how many there are.

But if retailers think they are going to get their hands on such priceless information, they are likely to be disappointed. The Competition Commission will aggregate all responses prior to publishing results, so it will be impossible to tell which authorities might be soft targets for growth-hungry multiples.

It also pledges to respondents: "We will not share your individual responses with anyone outside the Commission without your prior consent."