The CMA announced today that it has found 350 areas where there is an overlap between a Tesco and a shop with goods supplied by a Booker. Therefore customers could "face worse terms".
It also raised concerns about the potential for Booker to reduce its wholesale terms to the local stores "in order to drive customers to their local Tesco".
The CMA expects to publish its report before Christmas as the investigation will last 24 weeks.
Tesco announced the takeover at the end of January.