A group of charities has sounded the alarm over the growing use of buy-now-pay-later to buy goods.
Citizens Advice, a network of legal, money and consumer groups, said many users were getting into debt and struggling to pay for food and bills.
Buy-now-pay-later (BNPL) is increasingly popular among young people buying online, and at some High Street outlets.
However, Citizens Advice said many consumers regretted using it.
The charity network did not want to name BNPL firms, but called for tougher regulation.
It found that 45% of 18-to-34 year olds have used the payment option in the last 12 months.
The repayment option is often advertised at online checkouts as an easy way of splitting or delaying payments on items such as clothing or electronics, with incentives such as it being "interest-free".
However, the charity network said it can be a "slippery slope into debt".
The charity network said that, overall, 27% of UK adults have used these firms in the last 12 months, rising to 37% of disabled people and 45% of people with a mental health problem.
The average person was repaying £63 a month, but CA found almost two-in-five (5.7 million) who have used BNPL in the last year didn't think it was "proper borrowing" and six million didn't fully understand what they were signing up for.
'I've been barraged with calls from debt collectors'
One consumer who signed up for more than she expected is Sharonjit, whose story was cited by Citizen's Advice as an example of the trouble people can get into.
Aged 32, she bought £600 worth of clothes and used a BNPL firm to pay in instalments. She didn't receive the goods and cancelled her payment to the firm while she waited for the issue to be resolved by the retailer.
She said: "The whole thing has been so stressful. I'm constantly on edge. I've just been barraged with calls, emails and letters from a debt collector - all for buying some clothes online.
"The firm said they were referring me on to someone and I had no idea it was a debt collector. I had no idea buy-now-pay-later could [hit] my credit score.
"I've never had issues shopping this way before. But this time it's like as soon as something went wrong they've washed their hands of me."
And the charity warned that four-in-10 of those who've used BNPL in the last 12 months are struggling to repay.
It found a quarter of consumers regretted paying using these platforms, with consumers frequently saying they cannot afford repayments or are spending more than they expected.
Citizens Advice said firms must overhaul their checkout processes and improve affordability checks.
Both the Financial Ombudsman Service and Financial Conduct Authority had a greater role to play in the protection of consumers and regulation of the industry, it said.
Alistair Cromwell, acting chief executive of Citizens Advice, said: "Buy Now Pay Later borrowing can be like quicksand - easy to unwittingly slip into and much more difficult to get out of.
"It shouldn't be possible for people to sign up for credit without realising, and the fact this is happening so often signals that a drastic overhaul is needed.
"This industry more than trebled in 2020, and while these products work for many shoppers, the regulator has rightly recognised the potential for harm. It must ensure robust consumer protection keeps pace with changes in how we shop," Mr Cromwell said.
Several big - and smaller - names now operate in the fast-growing BNPL market, including Klarna, Clearpay, and Laybuy. PayPal launched a BNPL service last year.
However, firms stress that not all operate to the same standards.
Alex Marsh, UK head of the one of the most popular BNPL firms, Klarna, said: "There is now a variety of buy now pay later providers in the market and the findings in this report do not represent the experiences of the more than 12 million consumers who choose to use Klarna's interest free, fee-free services each year.
"We make it crystal clear for our consumers at check out that this is a credit product, we perform a credit check each time they use our service, and we are available 24/7 to support the small number of customers who unfortunately find themselves in difficulty."
Another firm, Openpay, said it had strict rules to protect consumers. Andy Harding, the firm's UK managing director of payments said: "Openpay has never run a marketing campaign specifically targeting young people and Openpay is never the default payment option at checkout - a practice that can lead to people using BNPL services unwittingly."
He added that Openpay has a financial hardship policy in place for any customers experiencing financial difficulty, including as a result of the coronavirus pandemic.