Almost one million motorists who used the Dartford Crossing could potentially be fined for having not updated their account details on the Dart Charge website.
National Highways figures show that 770,000 of the 1.7 million account holders revalidated their payment details upon request in July 2023.
Motorists needed to make the update after National Highways changed providers for the government-run Dart Charge service.
Drivers must pay the £2.50 charge by midnight the day after crossing the bridge, or can pay in advance by setting up an account so that it automatically docks money.
Around 4.5 million crossings are made between Essex and Kent every month and the vast majority of crossings are paid without issue, National Highways said.
A spokesman added: “All account holders were contacted in advance and prompted online, and to date 770,000 account holders have successfully updated their accounts.
“A number of things can cause a Dart Charge account to close, including inactivity or invalid payment details. In these instances several notifications are sent to advise the customer of the account status and detailed actions needed.
“However, we would encourage all account holders, particularly infrequent users to ensure their account details are up to date and there is a valid payment method on their account.”
The spokesman said that all penalty charges could be challenged by users if they do not agree.
Road users have told the BBC that they were unaware of the need to make the change and have now received fines.
A woman told BBC Essex said she had made the required updates but was charged in error.
She said: “I got a whole bunch of fines through. I was confused initially and then quite annoyed. It was hundreds of pounds.
“They changed system last summer and everyone was supposed to renew their account, which I did, but luckily I had an email saying I had set up my bank card.
“I was able to get them to cancel some of the fines but I lost money.
“I know lots of people who were given no notification and just had their account shut down.
“I spent an hour on the phone, the system has one job and it’s not doing it, it’s ridiculous.”
Another driver added that he was issued seven fines worth £332.50 in total after the same error happened to him.
