McLennan said the figures weren’t surprising given that around six million people don’t know or only have a vague idea of their monthly limit for call minutes, while five million are unsure of their text and data allowances.
This is despite seven in 10 people claiming to check their mobile bill at least once a month.
“You could be spending hundreds of pounds more than you need to on your mobile every year if you’re not on the right tariff,” McLennan said.
“If you regularly spend more than your line rental, check your bill to see where you’re incurring the extra charges as you may be able to save money by moving to a tariff with more minutes, texts or data.
“If you never go over your agreed line rental, it’s because you’re not using up your allowance so you may be able to save money by finding a cheaper tariff that reflects your usage.”
The Which? Mobile Study questioned 1,271 UK adults about their mobile phone contracts in November 2010.
Which? Mobile is a fully independent, free mobile switching service where consumers can compare thousands of tariff and handset combinations.