Ms Butler said the regulator would be conducting further work on its findings, including visits to principal firms, and would take "appropriate action" where businesses had failed to act on its recommendations.
The FCA urged the letter to be shared with company boards and threatened to consider the concerns raised in "future interactions" with recipient firms.
Appointed representatives undertake regulated activities under the supervision of an authorised firm which acts as their principal, with the principal accepting responsibility for activities and liabilities.
The regulator treats a breach or omission of its rules by an appointed representative as that of a principal firm itself, regardless of any additional commercial agreements between the two parties.
The FCA also reiterated no contractual arrangements could "remove the ultimate responsibility" of principal firms in ensuring customers of an appointed representative who have suffered detriment "receive appropriate redress".
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