One person in every ten admitted to having no idea how much debt they're in, according to a new study by Credit Expert.
96 per cent of Britons claimed they kept a close eye on their finances, but when questioned further more than a quarter of people surveyed weren't sure how much they had outstanding on personal loans.
More than half didn't know the APR on their credit cards and ten per cent said they had no idea of how much debt they had.
Jim Hodgkins, managing director for Credit Expert, said: "It's alarming to see that while almost the entire UK population think they are on top of their finances, many aren't."
"This research provides a worrying insight into people's perceptions of their finances. It is clear that many of us are not as familiar with our finances as we believe," he added.
Financial management proved most difficult for 18-34-year-olds, with nearly a quarter claiming they were bad with money.
The Financial Services Authority told the BBC that two-thirds of young people were not planning ahead financially.
