Heineken has been told to address fears that its purchase of around 1,900 pubs from Punch Taverns could affect competition in some areas.
The Competition and Markets Authority (CMA) said it had identified 33 local areas where pubs would not face sufficient competition after the deal.
Heineken, which already owns 1,100 pubs, must submit plans to the CMA by 20 June or face an in-depth inquiry.
However, the brewing giant said it would address the CMA's concerns.
"This decision by the CMA acknowledges that there are only a small number of local areas where competition may be diminished due to our acquisition of the pubs," said David Forde, managing director for Heineken UK.
"We are confident we can offer the CMA suitable undertakings to satisfy their concerns."
The preliminary CMA investigation concluded that the deal would not damage the chances of Heineken's competitors selling their own products, as the pubs being bought only make up 4% of the market.
It also felt that it was unlikely Heineken would reduce the choice of beer and cider available in the Punch pubs, as this could lead it to lose business.