Around 50,000 financial firms, including payday lenders, have come under tougher supervision to make sure they are not using customers as an "easy target".
The Financial Conduct Authority (FCA) has taken over regulation of the £200 billion-a-year consumer credit market from the Office of Fair Trading (OFT), which also means that anyone using a credit card, an overdraft or seeking help from a debt management firm will now have stronger protections.
The FCA has a remit to make sure businesses are putting consumers' interests at the centre of what they do.
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Firms will need to make sure they are providing services that meet customers' needs, that they are giving them enough information to make informed choices and that they treat struggling borrowers fairly.
The authority's chief executive, Martin Wheatley, said: "It's our job to make sure firms put their customers at the heart of their business and don't just see them as an easy target or a profit line."
The regulator said research indicates that around nine million people are living with serious debt.