One in five adverts from consumer credit firms, such as payday lenders, fail the City watchdog's marketing rules.
The Financial Conduct Authority (FCA) said yesterday that 108 of 554 ads it examined last month fell short.
In some examples, consumers were encouraged to hit the "apply" button before they had a chance to look up important information about a loan – a tactic that breaks consumer credit rules.
The FCA slammed other ads that targeted young audiences – by, for example, distributing branded colouring-in sheets with pamphlets for high-cost, short-term loans.
Other rule-breaking actions included a claim that a product would help repair credit ratings, and another that said a product would clear a customer's debt, when it just substitutes one debt for another.
Clive Adamson, director of supervision at the FCA, said: "It is particularly important in this sector that advertisements for financial products enable customers to make informed decisions. We think that more can be done to ensure that advertisements are fair, clear and not misleading."
Of the rogue adverts, 38 were produced by payday lenders, many of which failed to give enough prominence to risk warnings. The FCA said that 75 firms have so far responded by amending or withdrawing their promotions.