A software error is being blamed for an incident in which mobile phone users discovered they were being bugged by German secret squirrels.

According to reports last week, some customers of mobile phone operator O2 noticed an unusual phone number on their bills they didn't recognise and a call charge associated with the number.

When they tried to call the number they heard a recorded message telling them they couldn't use the number.

After further investigation, though, it was revealed that the number belonged to the German secret service. The number was showing up on people's bills because they were being bugged - and paying for it. Understandably, German police and the secret service were not too chuffed with this.

A spokesman for mmO2 blamed the incident on a "software error" and said it had now been fixed. Only a "very small number of individual users" were affected, he said. ®