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Some of BT's telephone exchanges had failed to recognise the 0845 numbers used by subscribers to connect, and instead registered them as dialling a free 0800 number.

Despite admitting that the whole affair has been due to its own mistakes, the telecom company still "needs to recoup the money", which is estimated to amount to around £5 million. The expensive oversight lasted for the first half of this year, but now BT wants its money back and is sending back-dated bills to 'lucky' punters.

BT provides Internet services to various ISPs as well as to its own (as was then) BT Internet. WHSmith and Tesco piggy-back BT's system, and some users of these ISPs are also affected by the mistake.