LONDON (Reuters) - Britain's competition regulator has referred supermarket Tesco's (TSCO.L) proposed 3.7 billion pound ($4.75 billion) takeover of wholesaler Booker (BOK.L) for a detailed investigation, granting a request from the companies to "fast track" the process.
Tesco, Britain's biggest retailer, and Booker announced the cash and shares deal in January and the Competition and Markets Authority (CMA) formally started a phase 1 review in May.
Last month, Tesco and Booker asked the CMA to move swiftly to a more in-depth phase 2 examination.
The CMA said on Wednesday it believed that in over 350 local areas where there was currently an overlap between Tesco shops and Booker-supplied independent grocery retailers, or so-called "symbol" stores, shoppers could face worse terms when buying products.
It said there were concerns that if the deal was cleared there was potential for Booker to reduce the wholesale services or terms it offers the stores it currently supplies, in order to drive customers to their local Tesco.
The figure of 350 areas under scrutiny was lower than initially speculated by analysts and media when the deal was announced.
Analysts at HSBC noted that previous reviews by the CMA, such as of Booker's purchase of Irish retail and wholesaler Musgraves in 2015, suggested that barriers to switching by symbol groups to alternative wholesalers were low.
"This offers some hope that once an in-depth review is carried out, remedies (required by the CMA) may be even lower than the initial 350," they said. HSBC has a "buy" stance on Tesco.
Booker supplies services to over 5,000 symbol stores, operating under the Premier, Londis, Budgens and Family Shopper brands.
It also supplies restaurants such as Wagamama and Carluccio's and operates the Makro cash and carry business.
Tesco runs more than 3,000 stores across Britain.
The CMA said other concerns were raised and considered in the initial probe, but it had not found it necessary to conclude on all of these given the referral.
The regulator will now assess whether the deal could reduce competition by conducting further research and analysis as well as seeking views and evidence from all those potentially affected by the deal.
The CMA's in-depth phase 2 investigation lasts 24 weeks. Its final report will be published before Christmas, following an earlier provisional findings report.
The transaction will be cleared if the phase 2 inquiry does not find it will reduce competition. If competition is seen to be affected, the CMA can either seek remedies or block the deal.
Tesco sees the deal as a new source of growth given Booker's role as a major distributor to the catering industry.
Some Tesco shareholders have criticized the transaction, saying it was overpaying and a distraction from its turnaround plan.
Tesco and Booker said they were pleased the CMA had accepted their fast track request.
"This merger has always been about growth, and we remain convinced that it will bring benefits for consumers, independent retailers, caterers, small businesses, suppliers and colleagues," a Tesco spokesman said.
"We look forward to continuing our constructive engagement with the CMA," added a Booker spokeswoman.
Shares in Tesco were down 0.2 percent at 171 pence at 1137 GMT, while Booker shares were down 0.5 percent at 189 pence.
Reporting by James Davey; editing by Kate Holton, Jane Merriman and David Evans