The lawsuit's implications are being felt across the industry

"We have opened the first pages of what will be a long and very sordid book," said California insurance commissioner John Garamendi.

The probe began a week ago when New York attorney general Elliot Spitzer sued broker Marsh & McLennan.

Mr Spitzer alleges that Marsh took illegal payments for steering clients to particular firms.

Shares in Marsh, the world's largest insurance broker, have collapsed since the Spitzer investigation was unveiled, and the firm is now renegotiating $2.8bn in bank financing.

This is going to be a long, long unhappy situation for the insurance industry John Garamendi, California insurance commissioner

Those of the companies alleged to have made the payments - including insurance giant AIG, Ace and Hartford Financial - have also suffered.

The firms concerned have said they will co-operate with the investigation.

Rules change

California has yet to identify which firms it will be looking into.

But Mr Garamendi made it clear that he was working closely with Mr Spitzer - and that his inquiry, already eight months old, would go beyond the corporate insurance which was the focus of the New York suit and into the consumer market too.

"The early indication is that it's an extremely serious breach of trust," he told reporters.

"Where (the investigation) leads we do not know. This is going to be a long, long unhappy situation for the insurance industry."

California, the largest state in the US and by some measures the sixth largest economy in the world, has also tightened up its rules on how insurance is sold.

In the future, firms will have to disclose any money they get for selling a product.