Tesco's acquisition of Mills Group was almost derailed by competition concerns over just six stores, the Office of Fair Trading has revealed.

The watchdog analysed all 76 stores involved in the deal at local level, its report into the December acquisition by Tesco's convenience store arm One Stop revealed. After drilling down through two stages of 'filters', it found the "competitive conditions" of six stores needed closer assessment as there would be fewer than four competing stores nearby if the deal was approved. The ­affected stores were in Richmond, Horncastle, Ruskington and Birtley, plus two in Marston Green, Birmingham.

The OFT visited the stores to assess local entry conditions, the proximity of competitors and constraints on rival fascias in the area, such as product offering, opening hours and store size. But it found "no realistic prospect of a substantial lessening of competition as a result of the proposed merger".

Third-party concerns that the deal would let One Stop get better supply terms were also dismissed because they would "ultimately benefit consumers", the report said.

The Association of Convenience Stores had called for the deal to be referred to the Competition Commission.