State-owned Lloyds Banking Group has been hit with a £117m fine by the Financial Conduct Authority (FCA), the "largest retail fine" it has ever issued.
The latest fine follows a £218m penalty issued by the watchdog late last year for Lloyds part in the international rate rigging scandal.
Today's fine is linked to payment protection insurance (PPI) complaint mishandling.
The regulator said Lloyds Bank Plc, Bank of Scotland Plc and Black Horse Ltd (together Lloyds) failed to treat customers fairly when handling PPI complaints between March 2012 and May 2013.
It said during the relevant period Lloyds assessed customer complaints relating to more than 2.3 million PPI policies and rejected 37%.
The FCA said firms are required to assess complaints impartially and can reject unfounded claims.
In March 2012, Lloyds issued guidance instructing complaint handlers that the overriding principle when assessing complaints was that Lloyds' PPI sales processes were compliant and robust unless told otherwise (the Overriding Principle).
In addition, it said, Lloyds did not notify complaint handlers of "known failings identified in its PPI sales processes" during the relevant period.
Some complaint handlers relied on the Overriding Principle to dismiss customers' personal accounts of what had happened during the PPI sale or to not fully investigate customers' complaints.
In some instances, Lloyds did not contact customers to enable them to give their account of the sale, the FCA said.
As a result of Lloyds' misconduct, a significant number of customer complaints were unfairly rejected.
FCA acting director of enforcement and market oversight Georgina Philippou said: "PPI complaint handling is a high priority issue for the FCA.
"If trust in financial services is going to be restored following the widespread mis-selling of PPI, then customers need to be confident that their complaints will be treated fairly.
"The size of the fine today reflects the fact that so many complaints were mishandled by Lloyds.
"Customers who had already been treated unfairly once by being mis-sold PPI were treated unfairly a second time and denied the redress they were owed. Lloyds' conduct was unacceptable."
|How customers lost out
• Complaint handlers justified the decision to reject customers' complaints on the basis that the sales process used by Lloyds was robust, when Lloyds knew there were significant sales process failures and mis-selling.
As a result of a substantial decline in the proportion of complaints upheld between March 2012 and October 2012, then-regulator the Financial Services Authority (FSA), began investigating the way Lloyds.
Following the FSA's intervention Lloyds removed the Overriding Principle from its PPI complaint assessment process and provided information on all sales process failings to complaint handlers.
The FCA said Lloyds has made significant progress towards the fairer treatment of customers in its general complaint handling operation and has established an extensive remediation programme to re-review or automatically uphold about 1.2 million PPI complaints, including those within the relevant period.
Lloyds has set aside a total of £710m to cover any redress due to affected customers.
Customers do not need to take any action. Those affected and due redress are being contacted directly. The FCA has appointed an independent skilled person to oversee the remediation process.
Lloyds announced in February 2015 that it had decided to freeze the release of shares in respect of deferred bonus awards from 2012 and 2013 for all members of the Group Executive Committee and for some other senior executives as a result of the FCA's enforcement investigation.
Lloyds agreed to settle at an early stage of the investigation and, therefore, qualified for a 30% discount. Were it not for this discount the FCA would have imposed a fine of £167,758,035.