IFA Brian Lentz is demanding compensation from Norwich Union after an admin blunder led to one of his client's details being leaked to a rival broker.
Lentz, of Hatfield-based Portfolio Insurance Consultancy, discovered that a client with a rebate-only appropriate personal pension was being approached by a member of the Countrywide network.
The network had also been getting commission on the policy since 1994.
Lentz is furious that his client's details have been released to Countrywide and says Norwich Union has breached the Data Protection Act.
He says: "How and why is Countrywide writing to one of my clients with the correct name, address and contract number? Why haven't Countrywide or Norwich Union picked this up before?
"I find it incredible that mistakes like this can be made."
NU has apologised for the mistake and refunded commission of more than Â£300 but has not paid interest or compensation. It blames the blunder on a "keying error" made when the policy records were updated. It is not making Countrywide refund the cash that it has received.
NU spokesman James Duffell says: "A complaint has been received and we are investigating it further. There was nothing underhand. It is a clear mistake on our behalf."
NU says it did not enter into correspondence with Countrywide about the customer and assumes that the policy details were taken from the commission statement.
Duffell did not know if compensation would be paid but says the client has not suffered.