The RAC is writing to customers after some key information in breakdown policy renewal letters was not as prominent as it should have been.
The Financial Conduct Authority (FCA) said the RAC had agreed to contact customers after failings relating to key information in its breakdown policy renewal documentation, with over a million customers potentially affected.
The regulator also warned general insurance firms it will take action over those who fail to properly implement rules introduced a year ago to make it easier for customers to shop around.
The rules, introduced in April 2017, require firms to clearly show the insurance premium a customer paid last year alongside their proposed renewal premium.
They also require firms to show a prominent, clear and straightforward message to encourage customers to shop around.
But the FCA said it has found that firms are still failing to properly implement the rules despite warning in October about failings.
It said some consumers may have lost out due to firms failing to get the rules right.
The RAC is writing to all affected members to clarify the key information, that was not as prominent as it should have been, to ensure that they understand it and are satisfied.
An RAC spokesman said: "While our breakdown policy renewal documentation did accurately contain all the necessary information, we recognise that some of the key information in our letters was not as prominently displayed as it should have been.
"We continue to be committed to giving members clear and fair information so they can make the right decision and are contacting those affected to ensure they are satisfied."
A spokeswoman for the Association of British Insurers (ABI) said: "Insurers pushed for these rules on renewal information because they want a market that operates in the best interests of consumers.
"Customers must have the same clear information whether they buy their cover direct or via a broker, bank or comparison website.
"While the vast majority of the insurance sector is abiding by the regulations, it is essential that any firm not meeting the standards works swiftly to address this."
Jonathan Davidson, executive director of supervision - retail and authorisations - at the FCA said: "It is simply unacceptable to see that some firms are still not being properly transparent with their customers a year on from the introduction of the rules.
"Firms failing to get this right may have led to consumers losing out as they do not have the right information to decide whether or not to shop around."We have already acted where we have seen particularly poor practice in firms and will continue to do so where we see firms not being transparent. As we said in October, we expect other firms to take notice of these issues, to look at what they are doing and to make sure they are getting it right."
Not putting the new rules into place for all products and customers
Getting previous premiums wrong
Leaving out the shopping around message or not presenting it in a way which draws the customer's attention
Failing to properly identify all customers who needed renewal information either because of system error or a mistaken interpretation of the type of customer included in the new rules
The FCA said it expects firms and senior management in those firms to take immediate action to ensure they are compliant.
It said any customer who feels that they did not receive the correct information from their insurance firm should contact them in the first instance.