Britain's financial watchdog has expressed its concerns about the scale of potentially problematic credit card debt and claimed more could be done to help struggling borrowers.
While consumers in default are "extremely unprofitable" for lenders, the watchdog noted that those with persistent levels of debt or who make minimum payments are profitable, and firms "therefore have fewer incentives to help these customers".
Two million credit cardholders are currently in arrears or have defaulted, according to the Financial Conduct Authority (FCA).
It estimates that a further two million people are in persistent levels of debt that some may be struggling to repay, and a further 1.6 million people only make the minimum payments on their debt each month.
Christopher Woolard, director of strategy and competition at the FCA, said more could be done to help the "significant minority" struggling with their credit card debt.
"This is a really important market in the UK. Around 60% of adults have at least one credit card, and there is an estimated £61 billion in outstanding balances," he said.
"Our study suggests that the market is working reasonably well for most consumers, with a range of cards on offer. However, for a significant minority who are in persistent levels of debt, the market could potentially work better."
The watchdog also called for the market to provide better information for those shopping around, and ensure borrowers can search the market without damaging their credit score.
Its final report on credit card debt will be published in spring 2016.
Responding to the report, Money Advice Trust chief executive Joanna Elson claimed the FCA was "absolutely right" to be concerned about borrowers repaying only the minimum amount. She called for such borrowers to be treated the same as those who actually miss payments.
"As the report highlights, credit card companies usually intervene effectively to help customers who begin to miss payments.
"A more consistent similar approach to customers who are routinely making minimum payments would be enormously beneficial.
"With borrowing on credit cards rising by around £300 million a month and household budgets likely to come under further pressure from rising rents and interest rate rises, now is the time to address this problem."