Aon's $30 billion bid for Willis Towers to create the world's largest insurance broker faces a full-scale investigation because of its complexity, two people familiar with the matter said.
The companies, which are the second and third-largest brokers globally after leader Marsh & McLennan, are looking to the deal to boost their pricing power.
Analysts said this will likely trigger deep regulatory scrutiny.
The insurance industry is facing a host of issues, with rising claims and new threats from the global outbreak of coronavirus and climate change.
Sources said the European Commission, which will open the probe following the end of its preliminary review on December 21.
Aon declined to comment.
A full-scale EU investigation takes about five months.
Aon did not provide concessions yesterday, the deadline for doing so in the preliminary phase to address EU competition concerns, the EU competition enforcer's website showed.
The two companies put together insurance contracts for clients that involve a number of insurance providers, for anything from airlines to large sporting events.
This is Aon's second attempt at acquiring Willis Towers. It dropped an earlier bid last year after media reports broke the news.