Brokers are calling on the Office of Fair Trading (OFT) to investigate claims that banks are using anti-competitive and unfair practices to sell insurance to business customers.
Insurance Times has received complaints from brokers that banks have been using "undue influence" to pressure commercial clients into buying insurance.
One broker said that "several" of his customers had been pressurised into buying insurance policies by bank managers who threatened not to grant credit facilities if they did not agree.
Simon Burgess, managing director of Burgesses, said the practice was tantamount to "blackmail".
Another senior broker said: "These are exclusive deals between the bank and its client. They are never written down on paper, but the client is under the impression that they will not be given credit unless they buy certain policies.
It is anti-competitive."
Broker Network chief executive Grant Ellis said the deals "are done in such a way that it becomes very difficult to spot."
Biba's head of technical services Peter Staddon said banks were guilty of "manipulating" the information given to SME commercial clients.
Staddon accused banks of "trying to sell customers a line so they have you by the short and curlies."
He said Biba wanted the OFT to look into the matter, adding that Biba believed banks should fully disclose how much profit is made on the back of such deals.
The OFT said it would need to receive a formal request to be able to investigate the matter.
When contacted by Insurance Times, HSBC, Lloyds TSB and Barclays said they were unaware of such misconduct and welcomed direct contact with specific examples.
In a separate move the OFT said it was to investigate a super-complaint made by the Citizens Advice Bureau (CAB) about the sale of payment protection insurance (PPI) to consumers.
The CAB said PPI was "failing" those who needed it most, adding:
"borrowers are often sold completely inappropriate policies when they take out credit agreements."