Insurer Admiral gave customers incorrect details
Tens of thousands of inaccurate insurance renewal notices have been sent to customers of Admiral.
All insurers have been ordered to include the previous year's premium in renewal notices, under regulations that began in April.
But the Financial Conduct Authority (FCA) said Admiral failed to show exactly what customers had paid.
The company has apologised and will contact affected customers who can cancel without a penalty if they wish.
New regulations, brought in by the FCA, were designed to help insurance customers to shop around and to make the rising or falling price of their insurance clearer.
All insurers have had to include the amount paid the previous year in any renewal documents for insurance such as home and motor cover.
However, Admiral - one of the largest insurers in the UK and a FTSE 100 company - failed to give accurate information.
It gave last year's quoted premium, before discounts were applied, rather than the amount that the customer actually paid, the FCA said.
Jonathan Davidson, of the FCA, said: "It is vital that insurers give customers the right information so they can easily compare premiums and make a choice whether to shop around.
"When introducing these measures, the FCA was clear that firms should be ready from April 2017. We will continue to keep a close eye on the industry's response to ensure firms are complying with the new rules and will take action where necessary."
Admiral, which has offices in Cardiff, Newport and Swansea, has apologised
"We are amending the way in which these customers are able to compare the price of the insurance policy being offered at renewal with what they paid the previous year," a company spokesman said.
"We are getting in touch with affected customers to ensure they are fully satisfied."
The company has refused to say how many people were affected, but the BBC understands it is tens of thousands. If affected customers choose to go to another insurer, they will be able to cancel without penalty and will have their premium refunded.
The FCA has given the company a deadline of August to ensure all renewal notices are correct.
Ian Hughes, chief executive of market research group Consumer Intelligence, said: "Insurance customers aren't quite getting the transparency the FCA envisioned on renewal disclosure.
"We are just over two months into the new era which was supposed to help shopping around and already there are problems with the likelihood of more to come."