An estimated four million UK mobile phone users have been charged £490 million for handsets they already own, a charity has warned.
Citizens Advice said EE, Three and Vodafone routinely continued to charge customers for handsets after they had paid them off in full.
It warned that customers were often unaware they were being charged for handsets after their contracts had ended, even though they owned the phone and only needed to continue paying for calls, texts and data.
The most common mobile contracts are bundled deals, which typically last for two years and do not include information about the cost of the handset.
A study by the advisory service of more than 700 bundled contracts found that consumers would pay more than buying the phone outright in 73% of cases, despite 55% of consumers assuming it was the cheaper option.
On average customers are overcharged £22 a month, but this was as high as £38 for high-end phones such as an iPhone or Samsung Galaxy, the charity found.
The study found vulnerable people were at higher risk of being overcharged, with older people twice as likely to be charged for a phone they already owned for longer than 12 months at an potential average cost of £264.
The advisory service is calling on the three companies to separate the cost of the phone from the mobile service - a practice already in place with other providers.