Aon, the world's second largest insurance broker, is to spin off its underwriting business into a separately quoted company.
Chairman Patrick Ryan said the new company, to be called Combined Specialty Corporation, would apply for its own listing on the New York Stock Exchange.
The entity has been valued at $2.1bn (£1.5bn) on a sum-of-parts basis by Credit Suisse First Boston.
But managing director Bob Rosholt insisted no market value had yet been put on the business, nor had the number of shares been decided.
The global underwriting businesses account for about one third of the present Chicago-based company.
Rosholt said revenues from the remaining brokerage side were around $5bn (£3.5bn), with underwriting turning over around $2.5bn (£1.8bn).
He said there was a “perceived conflict” between brokerage and underwriting, with competitor brokers not wanting to bring business to Aon's underwriting side.
The division of the company into two separate parts would resolve this problem.
Aon believes underwriting businesses will now be able to achieve double-digit growth, instead of its current single-figure performance.
Combined Specialty Corporation will take all the company's underwriting businesses around the world, including the UK.
At the end of last year Aon, whose total shares were valued at around $9bn (£6.4bn), compared with Marsh's $27bn (£19.2bn), announced a major restructuring.
This included cutting 3,000 jobs worldwide to deliver annual savings of up to $200m (£143m).
Rosholt said Marsh was only so much larger because it owned asset management group Putnam. “If you look at the brokerage side, we've been quite competitive,” he said.
Rosholt added that it was unlikely the remaining Aon business would go on a further acquisition trail.
He said: “The major part of the consolidation of the brokerage business is complete.
“Both Aon and Marsh made a lot of acquisitions in recent years, but the major part of that is done now.”