Brokers are increasingly inflating premium rates without insurers' knowledge so as to boost income, according to new research.
It is understood that a recent survey of compliance visits showed that instances of "grossing up" are rising.
A GISC spokeswoman confirmed that grossing up featured in many ongoing cases.
She said: "There is nothing wrong in an intermediary charging a fee and a commission for placing business, provided these charges are made clear to the customer. But charging additional amounts without customer consent is quite wrong."
Several leading brokers told Insurance Times that "grossing up" is so widespread within the industry that it "props up many brokers' bottom line".
One broker who wished to remain anonymous, said despite holding discussions with leading insurers, he found they have largely overlooked the issue.
Camberford Law chairman and chief executive Richard Sheikh said: "There are too many links in the distribution chain. We want brokers to take the lead and disclose commissions at every stage."