One in four have been hit by charges because they did not fully understand financial terms and conditions, according to a survey today.
Some 25% said their ignorance about money matters had cost them, while 40% think they know less about managing money than people did a generation ago.
Only 7% think the nation is well educated financially, while 71% thought a lack of financial education was responsible for debt, the study said.
But 70% of those questioned by price comparison website uswitch.com think personal finance is more complicated than it used to be.
Most people pick up their financial knowledge through trial and error, with 81% saying they learned their money management skills in this way, while 7% learned from their parents and 4% from their bank.
According to 95% of people personal finance should be taught in schools, and only 5% said they thought young people were well informed about financial matters.
Ann Robinson, director of consumer policy at uSwitch.com, said: "Our poor understanding of personal finance is costing us money and now looks to be getting worse with each generation.
"While our debt is increasing, our knowledge is decreasing - the situation is a ticking time bomb. The Government needs to start taking this seriously and should urgently reinstate plans to get financial education onto the curriculum."
