Aon's proposed $30bn (£21.7bn) all-share acquisition of Willis Towers Watson has been revoked due to concerns of an industry monopoly.
The deal, which was announced in March last year, would have seen the world's second- and third-largest insurance brokers form the largest global insurance broker and pensions consultancy.
Aon has confirmed there had been a mutual agreement to terminate the deal and end litigation with the US Department of Justice (DOJ).
Despite regulatory momentum around the world, including the recent approval of our combination by the European Commission, we reached an impasse with the DOJ."
Aon chief executive Greg Case (pictured) said: "Despite regulatory momentum around the world, including the recent approval of our combination by the European Commission, we reached an impasse with the DOJ.
"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."
Case added that Aon's respect for Willis Towers Watson "has only grown" from their experiences of attempting to come together in the last 16 months.
Willis Towers Watson chief executive John Haley said: "Our team's resilience and commitment are a source of pride and confidence. They have continued to bring to life Willis Towers Watson's compelling value proposition to better serve our clients in the areas of people, risk and capital.
"Going forward, our focus remains steadfast on our colleagues, our clients and our shareholders. We believe we are well-positioned to compete vigorously across our businesses around the world and will continue to introduce important innovations to the market. We appreciate and deeply respect all the Aon colleagues we got to know through this process."
Aon will pay a $1bn termination fee to Willis Towers Watson in connection with the termination of the business combination agreement.
The consultant confirmed Willis Towers Watson's proposed scheme of arrangement has now lapsed, and both organisations will move forward independently.