Customers using "buy now, pay later" services to purchase items from shops such as Asos and H&M have racked up an average debt of £538 each.
The number of customers using the controversial easy credit plans has grown by 1.6 million in 2021 so far, to 11.6 million, according to credit score firm Credit Karma. It estimates shoppers now owed buy now, pay later providers a combined £4.1 billion.
The credit is often free if buyers split their purchases into three monthly payments. Many firms, including Klarna, also offer a six-month and 12-month payment option, which will charge interest.
Campaigners and MPs have called for the sector to be regulated immediately. Last week the Treasury published a consultation on how greater scrutiny should be applied to the sector, which has boomed as online shopping has grown.
Ziad El Baba, of Credit Karma, said: “Buy now, pay later services can be a great tool for people who wish to make a purchase and break up their payments into smaller, more manageable amounts.
“However, borrowers must be cautious about how much debt they take on via buy now, pay later and make sure they're able to follow through with the terms of the arrangement. This is especially important since missing a payment can have a negative impact on your credit score, which can make borrowing more costly for consumers down the line."
Last week, a poll conducted for Laybuy, another buy now, pay later provider, found 63pc of shoppers wanted firms to conduct "hard" credit checks before allowing users to take out a loan. At present many companies perform lesser "soft" credit checks.
Shoppers also said buy now, pay later providers should share information about their loans to credit referencing agencies, as is the practice with credit cards and personal loans.
Currently, this data is not pooled between providers, meaning users can potentially rack up large debts with one buy now, pay later firm and then do the same with others. Some 71pc of customers said data should be shared so customers with debt issues can be prevented from borrowing more.
Credit Karma asked 2,000 adults about their use of the debt and users with outstanding debts owed an average of £538 to providers in the sector. Another piece of research from consultancy Bain & Co suggested a figure of about £200.
A spokesman for the industry's largest provider, Klarna, said: “Klarna offers a fair, transparent and sustainable alternative to credit cards which are the true drivers of debt.
"Unlike credit card companies who make their money when people miss or eternally defer a payment, we charge no fees or interest, provide a clear repayment plan, and our eligibility checks ensure we only lend to those who are able to pay back. Our average outstanding balance is £48 and we restrict the use of our services if a payment is missed to stop debt building up.”