The MP campaigning to cap the charges by payday loans companies has ramped up the pressure on the government ahead of a parliamentary vote.
Labour's Stella Creasy has proposed changes to the Financial Services Bill, currently going through parliament, that would give regulator the Financial Conduct Authority powers to force companies to curb charges on payday loans.
Interest rates in the payday loans industry can range from 222% to 16,000% for short-term loans, and one in three is taken out to pay off another loan.
But she said a cost cap would "set a ceiling" to help customers, and be more effective then implementing a cap on interest rates for every loan.
"With any loan you ever take out there would be a number beyond which interest rates, late repayment fees, admin charges it would never go. You set a ceiling which would be different on different types of loans.
"It would be higher than a basic interest-rate cap, but it would be more effective because when you talk to people in other countries that have got interest rate caps lenders just charging an admin fee to get round it."
In a statement Creasy said her amendment was "a key tool in the fight to tackle legal loan sharking".
"Fines will only have a limited impact on an industry which is making millions lending money at astronomical rates of interest to British families."
Speaking on BBC Radio 4's Today programme ahead of the vote on Tuesday, she said people deserve protection from "toxic" interest levels on loans. Creasy pointed to poll findings which showed 65% of people agree there should be a cap on what companies like Wonga can charge for credit.
However the Consumer Finance Association, which lobbies for the loans, said a cap will not help customers but will "simply make them unavailable."
Creasy has campaigned to curb the payday loan industry since she was elected in 2010, telling The Huffington Post UK earlier in May: "Caps on the total cost of credit wouldn't just protect consumers from these companies, they'd help our small businesses to manage their own finances - in two weeks time the government has the opportunity to do something about this and back my proposed amendment to their financial services bill."