The UK competition watchdog, The Competition and Markets Authority is probing the £403-million takeover of pub chain Punch Taverns by Heineken's as concerns around the deal increase.
The CMA said it was "considering whether it is or may be the case that this transaction, if carried into effect, will result in the creation of a relevant merger situation" and if so, "whether the creation of that situation may be expected to result, in a substantial lessening of competition".
Heineken struck a deal in December to acquire Punch Taverns with private equity firm Patron Capital, besting a rival bid from the pub chain's co-founder Alan McIntosh with a 180p-per-share offer.
The Dutch beer giant already owned 1,100 leased pubs across the UK and the deal would add another 1,895, while Patron acquired 1,329.
Concerns over the deal had been rife in the industry, with the Scottish Licensed Trade Association calling for the CMA to launch a probe.
The announcement comes a day after Heineken boss Jean-Francois Van Boxmeer praised British pubs as an "institution".
Meanwhile, the CMA said it would assess whether th deal could result in reducing competition and choice for customers.
The CMA would launch an in-depth merger investigation, which could last up to 24 weeks, if it found the deal could reduce completion. The two companies would need to address any competition concerns if the deal were to go ahead.
The Scottish Licensed Trade Association, welcomed the news. Its chief executive, Paul Waterson, said, ''Heineken is a global brewer, with very different priorities to their customers who often rely on hard earned local relationships to make their businesses work, The Guardian reported. We know from both Heineken's words and actions that they will give preference to their own products across their estate, and this is simply not fair for brewers, publicans or consumers. We look forward to receiving the findings of the investigation.''