The FSA today said it had referred one unnamed firm to its enforcement division for offering flawed investment advice to customers. Santander declined to comment to Reuters on whether it was the firm.

The enforcement action came after spot checks on the quality of advice available at six major lenders.

A spokesman for Santander told Reuters: “We are considering the findings in the context of the significant actions we took in 2012 to prepare for the post-Retail Distribution Review world.”

The FSA has uncovered evidence of incompetency among investment advisers at a number of banks and building societies – resulting in unsuitable advice in some cases – following a mystery-shopping exercise.

The regulator, conducting its first mystery shop since a similar exercise into payment protection insurance sales five years ago, carried out a total of 231 visits to six major retail banks.

While about three-quarters of customers received good advice, the FSA said, there was evidence of mis-selling.