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.

In particular, in 11% of its mystery shops, the evidence suggested the adviser gave the customer unsuitable advice. In 15% of visits, the FSA said it found examples of advisers failing to gather enough information from the customer to ensure the advice was suitable.

In 15% of mystery shops, the adviser had not properly ascertained the level of risk customers were willing and able to take while, in 13%, the advisers had failed to offer even basic financial recommendations, such as repaying unsecured debt.

The FSA said all six banks involved were cooperative and agreed to take immediate action, including the retraining of advisers and efforts to make substantial changes to their advice processes and controls for new business. They will also undertake past business reviews to identify historic poor advice and put this right for customers, the FSA said.

Clive Adamson, director of supervision at the FSA, said: “Mystery shopping allows us to understand what customers experience when they purchase financial products. This review shows that customers are not consistently getting the quality of advice on their investments that they should expect when visiting an adviser in a bank or building society.

“Whilst we are disappointed by the results of this review, we are encouraged by the action that the firms involved have taken to rectify the situation for their customers. Since this review took place, we have introduced new rules on investment advice which have increased the professional standard of the advisers operating in the market and have removed the potential for advisers to recommend products that pay the largest commission but may not be right for the customer.”