Staff at branches gave flawed advice and were not trained properly in dealing with customers' personal circumstances, according to the Financial Conduct Authority (FCA).
The FCA's report also said they did not get to grips with the level of risk their clients were prepared to take, ensure they had clear information and did not make regular ongoing checks on the viability of investments.
Tracey McDermott, the body's director of enforcement and financial crime, said the Spanish-owned high street chain had "let its customers down badly".
She added: "Customers trusted Santander to help them manage their money wisely, but it failed to live up to that responsibility. If trust in financial services is going to be restored then customers need to be confident that those advising them understand, and are driven by, what they need."
About 295,000 customers may have been affected between January 2010 and the end of 2012. Santander sold nearly 350,000 products worth about £7 billion during the three-year period.
Santander has apologised and says it will contact affected customers.