Almost three million mobile customers say they have been incorrectly overcharged by their network at a cost of nearly £64 million, a survey has found.
Each billing error costs consumers an average of £23, and they are unlikely to be identified and removed from the bill unless challenged by the customer, uSwitch found.
One in six mobile users say they have not checked their bill in the last six months, 47% because they assume it is correct, the poll found.
Ru Bhikha, mobiles spokesman at uSwitch, said: "While networks should be doing all they can to prevent billing errors, they can and do happen so it really does pay to be vigilant.
"Checking your bill should take just a couple of minutes. If you spot something that you think is a mistake, contact your provider to let them know - you might well find you're due a refund.
USwitch surveyed 2,005 UK adults in March.