It’s looking like a tough road ahead for insurance brokerage giants Aon Plc and Willis Towers Watson (WTW), whose proposed business combination is slated to undergo extensive scrutiny by the European Commission.

Citing people familiar with the matter, Reuters reported that the antitrust regulators of the European Union will be conducting a full-scale probe into the mega merger. It was noted that a comprehensive investigation lasts approximately five months.

“For Aon, our path forward to increase innovation and support clients is clear,” said Aon chief executive Greg Case in July. “Our Aon United book plan provides a proven roadmap, and the combination with Willis Towers Watson will substantially accelerate progress.

“Together we’ll be better for our clients on day one, driven by the complementary nature of our core businesses across solution lines and geographies, and will be better in the future driven by a shared commitment to analytics and increased ability to unlock new sources of value for our clients.”

Unifying Irish-domiciled and UK-headquartered Aon and WTW will catapult the merged organization to the top of the broker pile – ahead of US-based Marsh & McLennan Companies (MMC), which is currently the biggest worldwide.

Last year, Jardine Lloyd Thompson Group Plc (JLT) had to sell its global aerospace practice to Arthur J. Gallagher & Co. (AJG) so MMC could get the all-clear for its takeover of JLT.

The sale to AJG was in response to competition concerns raised by the European Commission, which at the time said the MMC-JLT merger would result in a significantly concentrated market.