Just Eat will face a full investigation over its £200m ($260m) plans to buy online takeaway ordering rival hungryhouse amid concerns the deal could curb competition.
The Competition and Markets Authority (CMA) said it would launch an "in-depth" probe over fears that "the merger could lead to worse terms for restaurants using either company".
Earlier this month the CMA asked market leader Just Eat for proposals to address the watchdog's concerns over industry dominance.
But the regulator said Just Eat had not offered any assurances, and so a full investigation will now go ahead, with a final decision scheduled for 2 November.
Both firms sell online ordering platforms to takeaway restaurants, which allows customers to choose from a wide variety of outlets in their area. Just Eat first announced its plans to buy hungryhouse, its biggest UK rival, in December.
The watchdog said: "Following its initial investigation into the merger, the CMA has found that the companies are close competitors because of the similarity of their service and their broad geographical coverage."
Last year Just Eat also struck deals to buy Canada's SkipTheDishes for £66m, and added businesses in Italy, Spain and Mexico.
At Just Eat's annual results in March it said over the coming year it expected to post sales of between £480m to £495m and underlying earnings ranging between £157m and £163m.