In January the regulator outlined concerns that firms were advising on pension transfers or switches to Sipps without assessing the advantages and disadvantages for customers of the underlying investments to be held within the new pension arrangement.
In a statement today the FCA said: "We carried out further supervisory work, including visiting some firms, to assess whether their business model complied with our requirements.
"Through this work, we continued to identify serious and ongoing failings."
It has taken action to stop a large number of firms from operating such business models and recently published two final notices on taking enforcement action against two partners in a firm, Andrew Rees and Timothy Hughes, who failed to comply with rules.
The FCA statement continued: "Customers have a right to expect an authorised firm to act in their best interests, yet the serious and ongoing failings found at firms have placed a substantial number of customers' retirement savings at risk.
"We believe pension transfers or switches to Sipps intended to hold non-mainstream propositions are unlikely to be suitable options for the vast majority of retail customers.

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"Firms operating in this market need to be particularly careful to ensure their advice is suitable.
"The failings outlined in this alert are unacceptable and amount to conduct that falls well short of firms' obligations under our Principles for Businesses and Conduct of Business rules."
The FCA said that where a financial adviser recommends a Sipp knowing that the customer will transfer or switch from a current pension arrangement to release funds to invest through a Sipp, then the suitability of the underlying investment must form part of the advice given to the customer.
The regulator stressed that if the underlying investment is not suitable for the customer, then the overall advice is not suitable.
The FCA said: "Generally speaking, we found very poor standards of advice."
More firms and senior managers may be referred to the FCA's enforcement division as a result of the assessment.