Borrowers would see their interest capped so that no-one pays back more than twice the amount of their original loan under a Labour Government, the shadow chancellor is set to announce.

John McDonnell will make the pledge to help tackle a "debt crisis" as he gives a speech at the party's annual conference in Brighton.

He will say that falling wages in real terms under the Conservative Government have created a "perfect storm" that forces families to take on debt to make ends meet.

Mr McDonnell will challenge 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.

If they fail to act, Labour will change the law when it wins power, he will say.

It comes as data shows that Britons' consumer borrowing has swelled to more than £200 billion for the first time since the crash of 2008, and overall household debt stands at a record of more than £1.8 trillion.

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 £14 billion on their cards.

Chief Secretary to the Treasury Liz Truss said: "We set up the Financial Conduct Authority which is ensuring credit card firms do more to help their customers clear debt and, from January, rip-off credit card charges will be outlawed.

"The best way to help people with their personal finances is with our balanced approach to the economy, which is creating more well paid jobs and cutting taxes for working people."

UK Finance chief executive Stephen Jones said consumer credit helped drive economic growth.

He said the FCA had looked considered whether a cap on interest charges should be introduced, but concluded that there were preferable alternatives which the industry will implement when a consultation is concluded later this year.