The price comparison site found that a third of UK consumers (32%) have faced additional costs over the past year, totalling £489m.

Those affected clocked up an average of £38.90 over and above their regular payment during the past 12 months.

Roaming charges emerged as the most costly surprise, with those affected reporting an average charge of £24 over the course of the year. This was closely followed by international calls at £21.90 and in-game currency at £21.80.

The analysis found that more than one in 10 (11%) bill-payers were charged for calling a premium rate number, while 10% exceeded their data allowance, 8% were charged for making international calls, and 7% picked up international roaming charges.

Despite so many people racking up unexpected charges, almost half of mobile users (47%) say they don’t check their bill every month. A fifth (19%) admit to never checking their bills.

A fifth (22%) of those who have spotted a nasty surprise on their bill attribute it to someone else who had access to their phone. Over half (56%) of these people blame their tech-savvy children, with kids responsible for adding £60m to their parents’ mobile bills.

Ru Bhikha, telecoms expert at Uswitch.com, said: “Once you’ve got your new phone, it can be too easy to ignore the monthly direct debits — and assume the amount you’re billed won’t change. But failing to keep an eye on what you’re being charged can leave you open to nasty and expensive surprises, especially as more and more of our communications are found online.

“At a time when millions of Britons are watching what they spend in other areas, it’s a surprise that so many are taking their eye off the ball when it comes to their phone. Keeping on top of your mobile costs is a good place to start when watching your pennies, and you should get into the habit of checking your bill every month.

“If you find yourself regularly facing unexpected charges, it’s worth having a chat with your network provider. They might have another tariff which suits your needs better, or they could tell you exactly how you’re incurring the extra costs and give you pointers on what to avoid.”