The Royal Bank of Scotland (RBS) has been slapped with a £28.6m Office of Fair Trading (OFT) fine for breaching competition rules.
The OFT probe began following a tip-off from Barclays Bank that RBS had disclosed confidential information about the pricing of loans.
Rules were broken between October 2007 and February or March 2008, the OFT says.
The information concerned the pricing of loans to large professional services firms, such as solicitors, accountancy, and property companies.
OFT senior director of cartels and criminal enforcement Ali Nikpay says companies disclosing confidential pricing information face "substantial" penalties.
"It is important that companies operating in the UK understand the seriousness of such conduct and ensure effective competition compliance throughout their organisation," he said.
Barclays informed the OFT of the breaches, an admission which, under OFT rules, should gain it immunity from prosecution.
The OFT said that provided Barclays continues to co-operate it "is not expected to pay a fine in this case".
RBS's fine was reduced from £33.6m to reflect the bank's "admission and agreement to co-operate".
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