Linklaters has represented Clydesdale Bank in a Financial Conduct Authority (FCA) investigation into "serious failings" in the lender's payment protection insurance (PPI) complaint handling processes, Legal Week can reveal.

The investigation culminated in a fine of more than £20m for Clydesdale today – the largest fine the regulator has imposed in the ongoing PPI scandal.

The financial regulator found that in mid-2011 Clydesdale implemented policies that meant its PPI complaint handlers were not taking into account all relevant documents when deciding how to deal with complaints.

The FCA also found that between May 2012 and June 2013 Clydesdale provided false information to the Financial Ombudsman Service in response to requests for evidence of the records Clydesdale held on PPI policies sold to individual customers.

It found that, as a result of Clydesdale's conduct, 42,200 PPI complaints decided between May 2011 and July 2013 may have been rejected unfairly and up to 50,900 upheld complaints may have resulted in inadequate redress for customers.

Linklaters does not have a spot on Clydesdale's panel, but is understood to support the bank in matters that involve dual-reporting and corporate deal-making.

In 2011 Linklaters won a spot on Clydesdale parent company National Australia Bank's UK legal panel.

Linklaters did not secure a place on the FCA's inaugural panel in 2013, a seven-strong roster formed of Ashurst, Clifford Chance, DLA Piper, Eversheds, Hogan Lovells, Macfarlanes and Pannone.

Linklaters and Clydesdale declined to comment.