The departure of one of Marsh's leading marine specialists may have cost the broker the world's most valuable marine account, according to industry sources.
Aon this week snatched the Carnival Cruise fleet account from rival Marsh, which had held it for more than 12 years.
The coup comes just weeks after Steve Beslity left Marsh to join the management team at Benfield's newly-formed energy, marine and power division.
Beslity was global marine practice leader at Marsh until March 2005 and is understood to have led negotiations with Carnival over the past few years.
A source said: "Steve was massively influential on the Carnival account. He had forged a strong relationship and when he went, the business followed. There is no doubt that his departure would have affected the decision to place the risk with Aon."
Carnival pays around $65m a year in premiums to obtain P&I and hull cover for its 77 cruise ships, valued at £23bn. The fleet includes P&O Princess and Cunard ships, as well as the 1,132ft Queen Mary 2.
The risk is predominantly placed in the London market, but is also underwritten in the US, Norway, France, Italy and other markets.
Another prominent theory is that Carnival felt uncomfortable placing the business with Marsh in the light of New York attorney general Eliot Spitzer's clampdown on the broker's use of contingent commissions.