The mega-merger that would have created the biggest insurance brokerage in the world has been abandoned, with Aon paying Willis Towers Watson (WTW) a $US1 billion ($1.36 billion) termination fee.

The companies revealed the merger proposal in March last year but last night “agreed to terminate" the business combination agreement after being unable to resolve US regulator concerns.

“Despite regulatory momentum around the world, including the recent approval of our combination by the European Commission, we reached an impasse with the US Department of Justice,” Aon CEO Greg Case said.

“The DOJ position overlooks that our complementary businesses operate across broad, competitive areas of the economy.

“We are confident that the combination would have accelerated our shared ability to innovate on behalf of clients, but the inability to secure an expedited resolution of the litigation brought us to this point.”

WTW CEO John Haley says the business is well-positioned to “compete vigorously” around the world.

“We appreciate and deeply respect all the Aon colleagues we got to know through this process,” he said.

The organisations will move forward independently, providing further financial updates and outlooks on Q2 earnings on July 30 (Aon) and August 3 (WTW).