The head of the Financial Conduct Authority, Martin Wheatley, said the regulator wanted to "take out" unscrupulous payday lenders.
He told Radio 4's Today programme.
Our concern has been that parts of this industry, not all, loan without doing any affordability checks and then loading costs onto people who simply cannot pay, and that's the part of the industry that we really want to take out, we want them to improve their standards or leave the industry.
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The man, in his thirties, was found with stab injuries on the upper deck of a bus in Handsworth, Birmingham, on Monday night.
Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.