Heineken's £403m acquisition of pub group Punch is to be investigated by the Competition and Markets Authority (CMA).
A deal was agreed by Heineken in December, backed by investment firm Patron Capital, to add 1,900 Punch pubs to its 1,100 strong Star Pubs and Bars portfolio.
At the time of the merger's announcement, David Forde, managing director of Heineken UK, said that the deal would be good news for drinkers.
"Our proven track record of success demonstrates that well invested and well run pubs in the leased and tenanted sector can thrive," he said.
"Today's development is good news for pub-goers across the UK who will see the benefit of better pubs in their communities."
The CMA has now announced that it will assess whether the deal is likely to reduce competition and choice for customers.
The first phase of the investigation will last up to 24 April 2017.
If the CMA believes that competition will be reduced as a result of the deal an in-depth merger investigation will be triggered, lasting up to 24 weeks.