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The consumer watchdog reports that 3 is the worst offender; of its 50 surveyed customers, a third said that they had had a problem with their handsets in the first 12 months.

The UK's four other network operators were little better, with an average 15 per cent fault rate. Tesco and Virgin Mobile proved the most reliable providers. Among handset manufacturers, Nokia and Samsung proved the most reliable, at less than 10 per cent, with Motorola and Sony Ericsson faring worst.

'A one in seven chance your phone's going to develop a fault is way too high,' said Malcolm Coles, editor of Which?. 'Retailers, who should be bending over backwards to help

3 defended itself, saying Which?'s survey of just 50 of its three million customers was not representative, adding that the advanced technology meant its record could not be compared to the other networks'. Which? made that point in its report, suggesting that the 'hi-tech' products may have experienced teething problems.

However the industry said that phones cannot be compared to other electronic products as they are more prone to knocks.

'You don't carry your TV around in your handbag or pocket every day,' said a spokeswoman for the Carphone Warehouse.

Which? claims that problems are exacerbated by the way that they are dealt with. By law, if a phone develops a fault within six months and the shop can't show it was the customer's fault, the shop must sort out the problem, the watchdog explains, but it found that over a quarter of people questioned were unhappy with the handling of their complaints.

As a result, it has created complaint letter templates which can be copied from trial.which.co.uk.