Aon is preparing to submit a bid for Willis Towers in the coming weeks and the two companies have also held preliminary talks, the report said.
With an acquisition of Willis Towers, Aon could surpass Marsh & McLennan as the world's largest brokerage by revenue, according to data compiled by Bloomberg.
The reported takeover approach comes as Marsh & McLennan Companies (MMC) waits to close its £4.9bn ($6.4bn) takeover of JLT.
With a market capitalisation of well over $23bn, a Willis takeover would be significantly larger than the JLT purchase, with the fusion of two of the top three brokers likely to raise more competition concerns.
The insurance broking industry has consolidated significantly over the past several years, with large brokers snapping up numerous smaller players. The sector has also proved attractive to private equity.