The AiiB's campaign against poaching of its members' business is snowballing.
The broker body has won support from the Consumers' Association in its quest to stop high street lenders using privileged information to win intermediaries' customers.
Intermediaries complained about sharp sales tactics of Nationwide and Abbey National staff.
Now the AiiB has unearthed yet another case of alleged sharp sales practice.
Anne Kipping, director of Bedfordshire-based intermediary Flitwick Insurance Services, has highlighted a case from March 1997.
One of her customers went into the Flitwick branch of the Woolwich near Bedford, to get a cheque for his motor insurance.
But, instead of issuing the cheque, the cashier offered a rival quotation.
When told of the incident, Kipping complained to the Woolwich in the belief that the 1984 Data Protection Act had been breached.
She told Insurance Times: "The customer had a cheque made out in our name and the Woolwich took advantage of that privileged information."
The bank claimed it had acted within the law in the interests of its customers by bringing its products to their attention.
Mrs Kipping decided not to pursue the case in the courts because of the cost.
AiiB chairman Mike Slack said he would be seeking the view of the Data Protection Registrar on whether personal information had been obtained improperly.