Barclays was fined 8.4 million pounds (U.S. $10.2 million) for failing to provide retailers with complete information about the costs of the card services it supplied, which prevented them from shopping around for cheaper deals and passing the benefits onto customers.

The U.K.’s Payment Systems Regulator (PSR), which oversees methods of bank transfer, announced its findings Thursday that Barclays did not provide retailers with transaction information required under Article 12 of the EU Interchange Fee Regulation 2015/751. As a result, retailers were unable to easily understand the transaction fees associated with accepting certain types of card payments.

Since Barclays did not properly inform retailers about how fees were calculated or structured, or even how much they were being charged, they could not compare the bank’s charges with other card services providers to find cheaper deals or negotiate better terms with Barclays, according to the regulator.

The PSR’s investigation, which began in November 2018 after it identified failures as part of its regular monitoring activity, found Barclays failed to comply with the regulation from December 2015 to December 2018. During this time, the bank processed a third of all card payment transactions in the United Kingdom, meaning thousands of retailers and transactions were affected.

Barclaycard Payments, the payments processing business within the bank, was undertaking a large-scale system upgrade when Article 12 was introduced, the PSR noted. As a result, there was a delay in the bank being able to make available transaction-level reports.

In a statement, Barclays said it now makes the full information available to retailers and it fully cooperated with the PSR’s investigation. The bank’s fine was reduced by 30 percent from £12 million (U.S. $14.6 million).

“Barclays’ failure to be transparent with retailers about the fees they pay for card services meant retailers could have been missing out on better deals,” said PSR Managing Director Chris Hemsley in a press release. “Where we see banks failing to comply with the Interchange Fee Regulation, we have powers to intervene to make sure these important rules are followed so retailers can make informed decisions about the services they use.”

In May, NatWest was fined £1.82 million (then-U.S. $2.2 million) in the PSR’s first enforcement case for noncompliance with the regulation after the banking group overcharged for interchange fees on credit cards.