More than six million people have had their credit card limits increased without their consent in the last year, research from Citizens Advice suggests.
The organisation also found that consumers struggling financially were more likely to given extra credit – adding to concerns about the build-up of household debt.
Citizens Advice said lenders were “actively pushing” more borrowing without considering who could afford it.
It has been calling for a ban on unsolicited credit limit increases to help prevent people from accruing debts they cannot pay back – something it says puts individuals as well as the wider economy at risk.
Industry body UK Finance said providers were committed to responsible lending.
According to Citizens Advice, 28% of credit card holders – equivalent to 8.4 million people – received a credit limit increase over the past year.
But only 23% of those actually asked for the rise, with the remaining increases initiated by lenders.
Video: October: Young people anxious about their rising debt levels
The study also found that 32% of credit card holders who were not confident they could pay back their debts were given a credit increase.
That was a higher proportion than the 23% of those who were confident having limits raised.
On average, credit card holders were given rises of £1,481 without being asked – with one in ten receiving increases of £3,000 or more.
The research was based on a ComRes survey of more than 1,300 credit card holders.
Citizens Advice gave the example of a woman who had asked for help after building up credit card debt of £3,500 that she was unable to pay back.
She initially had a credit limit of £500 to pay unexpected bills – but the limit was extended when she reached it, and then extended multiple times thereafter.
Citizens Advice chief executive Gillian Guy said: “Few consumers support unsolicited increases and our research shows that they make people’s debt problems worse.
“The Chancellor must step in to prevent credit card companies weighing people down with unwanted debt – particularly when they are already struggling to keep their heads above water.”
Richard Koch, head of cards at UK Finance, said: “The industry has come together to voluntarily agree new protocols to ask customers whether they would prefer to opt out or opt in for any credit limit increase offers.
“Furthermore, the customers who the Financial Conduct Authority and Citizens Advice are most concerned about will be excluded from receiving any such offers.”
The study comes amid concerns from the Bank of England about the scale of unsecured lending.