Tesco and Booker have asked the competition watchdog to "fast-track" its investigation into their £3.7 billion merger, confirming that the deal is expected to face an in-depth probe.
The Competition and Markets Authority (CMA) had been expected to complete the first phase of its investigation by July 25, but the two firms have requested it speeds up referral to a more in-depth second phase.
In a stock market announcement, Tesco said: "Following continued constructive dialogue and further to the commencement of the CMA's Phase 1 review on 30 May 2017, we have now requested that the CMA uses the 'fast-track' process to allow it to move more quickly to examining the merger through a detailed Phase 2 process.
"We expect the CMA to issue an early decision to refer to Phase 2 within the next two weeks."
If the CMA proceeds with a fast-track procedure, it would expect to issue a phase 1 decision in the next 10 working days.
The statutory timetable for phase 2 would be 24 weeks, the CMA said.
The watchdog is assessing whether the deal, announced in January, could reduce competition and choice for shoppers.
Booker is the country's largest wholesaler and owns Londis and Budgens as franchised outlets.
Tesco has faced criticism from investors over the move, with some shareholders branding the takeover tilt a "distraction" and urging the Big Four grocer to scrap it.