More than nine million Brits could be forking out for mobile phone handsets they have already paid for by not switching tariffs at the end of their contract.
Nearly three-quarters of people on pay-monthly contracts opt for a deal which comes with a 'free handset', which effectively means their tariff is inflated to include the cost of the phone.
However, a third of mobile users whose contracts are up for renewal haven't bothered to switch to a new deal, according to research from uSwitch.com, and this means they are overpaying for the cost of the phone.
If they switched to a SIM-only or a new pay-monthly contract, Brits could save a staggering £1 billion - or £104.40 each - over 12 months, the price comparison site found.
Ernest Doku, telecoms expert at uSwitch.com, added: "Sometimes it pays to spend a small amount upfront and go for a deal offering a lower line rental, rather than opting for a free phone with a hefty amount to pay each month."
Consumers are also regularly exceeding their data allowances, with 23% of users paying £7.60 a month in out-of-tariff data charges. Annually, people are wasting more than £885 million by not being on a contract that is most suitable for them.
The average Brit uses just 39% of their monthly minutes, while 21% estimate that they use less than a fifth of their talk allowance in a typical month, meaning people could be losing money by not switching to a more appropriate deal.
While two-thirds of people (67%) have switched mobile deals at least once, only 33% are with the same network they were when they first had a mobile.
A fifth (18%) said it has too much hassle to swap networks and 10% said the process was too complicated but the majority (64%) said they were happy with the service they received.