Mobile customers are being "ripped off" by operators who continue to charge them the full price of their contracts even when they have paid off the cost of their phone, says Which?

Some are potentially overpaying by more than £400 a year, according to the consumer watchdog.

Last year Ofcom asked operators to reduce prices to out-of-contract customers from February this year.

Operators, however, said they were offering customers plenty of choices.Many customers choose to get a new smartphone through a monthly bill contract, effectively paying off most of its cost over a year or two.

'Worst offenders'According to Which, the worst-affected were customers of Three, where about four in 10 customers whose contracts ended in the last six months claimed they saw no price drop.

Two in five EE customers and three in 10 Vodafone users said the same.On the other hand, O2, Tesco Mobile, and Virgin Mobile told customers that when their contracts ended their bills would reduce to the best available air time deal.Examples are given by Which?