Biba has threatened a "distribution war" against direct insurers in response to brokers' growing anger over anti-broker marketing messages.
The trade body said that its members were becoming increasingly frustrated by the constant criticism of brokers in direct insurers' advertising and marketing material.
It warned that it would be prepared to "fight back" and begin an aggressive pro-broker campaign unless the ABI used its influence as the insurers' trade body to "control" the messages being issued.
Peter Staddon, head of technical services at Biba, said: "The direct writers are putting out information that does the industry a disservice. We don't want unfair accusations being levelled against brokers."
Staddon criticised the direct writers' constant attacks on brokers' commissions when they themselves spend millions on acquisition costs.
He said direct writers' cover is "pruned", allowing them to charge lower premiums, and that average claims pay-outs are lower than those secured through brokers, both of which are not pointed out. He added that the majority of calls to Biba's helpline are in respect of direct writers.
Staddon said: "[Direct writers] are members of the ABI, but the ABI is not controlling its members. Either the ABI stops this happening or the distribution war will begin."
He said Biba would issue its own messages promoting brokers and explaining why direct insurers should not be used. "We will employ the tactics that direct writers are using," he warned.
Biba technical services manager Graeme Trudgill said: "Biba has a lot of influence in the national media. We are relaunching the 'find a broker campaign' and will talk about the benefits of using a broker and not going direct. If the ABI doesn't do anything we will have to fight."
Direct insurer First Assist, which recently wrote to motorists claiming brokers could be raking in up to 20% in commission, defended its marketing messages, with a spokesman describing them as legitimate. "We're building a brand in the non-standard market, a sector which tends to be badly served by other providers. There is more chance higher commissions will be charged."
The spokesman denied the letters were unfair in suggesting that using a broker would result in more expensive cover. He also said it was not an issue that direct insurers tended to spend more on marketing than brokers - a cost which would be probably be passed onto the customer.
"Brokers will have their own marketing spend and I would suggest that the AA spends far more than us."
He added that brokers "needed to be upfront" about commissions they were charging.
Justin Jacobs, ABI head of liability, motor and risk pricing, said: "We work closely with Biba when there is a public market issue, such as tackling uninsured driving.
"The ABI is in favour of free and fair competition - which is in the public interest - within the insurance industry, however it is sold."