Bradford & Bingley will no longer deal with third parties representing customer endowment complaints.
The bank says its customers may be unaware of the free complaints service already available to them through the Financial Ombudsman Service.
It will however deal with people representing customers with special circumstances or those customers insistent on using third parties.
From August 1, if B&B receives a complaint via a claims management company, it will write to the customer informing them they can complain for free and provide them with relevant information to do so.
B&B spokeswoman Nickie Aiken says: “Customers are being left financially disadvantaged by using such firms. There is no reason for customers to have to resort to these firms. The complain service we provide and that of the FOS are both straightforward and free.”