A Labour government would cap credit card interest so no one pays back more than twice the amount of their original borrowing, Shadow Chancellor John McDonnell has announced.
In his keynote speech to Labour's annual conference in Brighton, Mr McDonnell will accuse the Government of creating a "debt crisis" which is becoming a threat to the economy.
He said three million people trapped in persistent debt with credit cards in the red by a total of £14billion.
He will call on ministers to apply the same cap on credit card debts as on pay-day loans, limiting interest and charges to 100% of the amount borrowed.
He will promise that if they fail to act, Labour will change the law when it wins power.
The Financial Conduct Authority estimates that more than 3 million credit card users - a tenth of the total - are in persistent debt, handing over more in interest and charges than in paying down their loans over the past 18 months.
The average amount outstanding for those in persistent debt is £3,464, meaning that these people between them owe around £14billion on their cards.
Wider consumer debt, including credit cards, personal loans and overdrafts, has reached more than £200billion for the first time since the crash of 2008 and overall household debt stands at a record of more than £1.8trillion.
Mr McDonnell will say: "The last seven years of Tory economic failure has created the perfect storm, as wages have fallen behind, more and more families are being pushed deeper into debt.
"Under pressure, the Government has been forced to cap interest payments on payday loans.
"But more than 3 million credit card holders are trapped by their debt. They've paid more in interest charges and fees than their original borrowing.
"The Financial Conduct Authority has argued for action to be taken on credit card debt as on payday loans.
"I am calling upon the Government to act now to apply the same rules on payday loans to credit card debt. It means that no-one will ever pay more in interest than their original loan.
"If the Tories refuse to act, I can announce today that the next Labour government will amend the law."