The Competition Commission was reportedly "seething" this week after being forced to stop Tesco developing the site of a former Co-op supermarket in Slough. The store is the subject of a parallel investigation to the main groceries inquiry and this week was the first instance of the Commission using its full powers since either inquiry began. Tesco had erected the metal frame of the store and building work was 50% complete before it was ordered to stop work on Monday. A source close to the inquiry told The Grocer it was "unfortunate" Tesco had decided to proceed when the outcome of the inquiry was in the balance and "the inquiry team was seething" as it would "lead to considerable difficulties if the process had to be reversed" . The Commission may still instruct Tesco to tear down the building, he added. However, a spokeswoman for Tesco said work had only continued because the Commission and the OFT had specifically told it that it was free to do so. The Tesco case has taken on greater significance, as the Commission has indicated that it will be first to feel the weight of its wider thinking on the grocery market that is now taking shape as the main groceries inquiry concludes. The Commission is set to reveal its provisional findings in the wider inquiry in September. The Tesco case has a statutory deadline of 3 October. Tesco, which already has a large Extra in Slough, bought the Co-op site in 2004 but was ordered by the OFT to find a suitable one-stop grocery provider to purchase it. Having failed to do so, the OFT referred the case to the Commission in April. Meanwhile, the Commission has further upset Tesco this week by again rejecting its local market model for determining effective competition. Despite the supermarket chain's 28-page response to the Commission's Emerging Thinking claiming that the Commission's definition of local is up to three times smaller than it should be, a Commission working paper on quantitative analysis rejected Tesco's highly technical SSNIP test theory. "There have been a lot of exchanges with Tesco, but we still see the area for local competition to be a 10-15 minute drive," said a Commission spokesman.