Aon and Willis Towers Watson (WTW) have stressed they are well placed to continue growth independently after dropping their proposed $US30 billion ($40.8 billion) merger due to US regulatory opposition.

The dropped deal will also mean the proposed sale of assets to Gallagher, aimed at securing regulatory approvals, will not proceed.

Aon CEO Greg Case says the firm’s ability to innovate on behalf of clients remains “unrivalled” and a new Executive Committee has been announced to advance a four-pronged blueprint.

“Our Aon United operating model has been further strengthened through the combination planning process and we're moving forward at speed without the overhang of regulatory uncertainty or distraction of restructuring,” Mr Case said.

The firms argued that the Department of Justice had overlooked the complementary nature of businesses that Aon and WTW 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,” Mr Case said after the deal was dropped.

The Department of Justice filed a civil antitrust lawsuit on June 16 to stop the merger, raising concerns over the reduction in competition from combining the second and third largest brokers, replacing a “Big Three” with a “Big Two”.

“This is a victory for competition and for American businesses, and ultimately, for their customers, employees and retirees across the country,” Attorney General Merrick Garland said after the deal was halted.

Aon will pay WTW a $US1 billion ($1.36 billion) termination fee as a result of the decision against continuing with the takeover.

"Going forward, our focus remains steadfast on our colleagues, our clients and our shareholders,” WTW CEO John Haley said. “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.”

Gallagher says that while the proposed acquisition of certain Willis Towers Watson brokerage operations has halted, its “decades-long tuck-in merger program remains a proven strategic growth engine”.