Brokers are a gateway to lenders, and are particularly used by borrowers who go to payday lenders.
But the City watchdog said the brokers often failed to give full details of their fees or to explain why these were being charged.
The brokers also now have to put their name on their adverts, and explain they are not the lender.
“That we have had to take these measures does not paint this market in a particularly good light,” said FCA chief Martin Wheatley.
“I hope that other firms will take note that where we see evidence of customers being treated in a blatantly unfair way, we will move quickly to protect consumers from further harm.”