Aon has accused Heath Lambert and Marsh of establishing broker fee models which "will not work".
The broker said it would not impose set fees for services on insurers, but will make its charges clear by adopting the Beazley working party's proposed changes to the London Market Principals (LMP) slip.
The changes will allow underwriters and brokers to see how much commission is being paid to the broker and to the London market.
Aon said it had already begun to implement the change within its operations and accused Marsh and Heath Lambert of having "their heads buried in the sand".
But Heath Lambert and Marsh rejected claims that their models were unworkable.
Heath Lambert said it was committed to its proposed 1% flat rate commission.
A spokesman said: "We already have widespread support within the company market. Our active focus now is on discussions with the Lloyd's market on exactly how the 1% will be recognised."
Marsh said it supported the Beazley working party efforts. A spokesman said: "We have come to agreement with the London Market Association (LMA) on details of a London market slip that will fully disclose any commission and/or charges."