The BT employee, aged 29, is believed to have once worked at a malicious calls bureau; one of the other detainees is understood to be a private investigator.
Scotland Yard emphasised that yesterday's arrests were not connected with allegations that the Princess of Wales made nuisance phone calls to Oliver Hoare, the millionaire art dealer.
The arrests follow an investigation by police into allegations that private detectives are routinely sold information by BT staff and obtain other confidential data through deception and by electronically breaking into computer records.
The man from BT who was arrested is understood to have worked in the accounts department at the Delta Point administrative centre in Croydon, south London.
The three men were arrested in Croydon and Sutton, south London, and were being held at a police station in south-west London.
A Scotland Yard spokesman said: 'They were arrested in connection with obtaining confidential information from BT illegally.'
British Telecom's staff would have played a part in monitoring Oliver Hoare's phone line and the subsequent tracing of the source of the calls. But BT has strongly denied that the leak may have come from one of its workers.
Chris Brogan, the managing director of Security International, a detective agency in London, and the former president of the Council of International Investigators, said yesterday that about six out of 10 agencies had contacts with BT employees.
He said: 'Detectives target someone, such as an engineer, who works for BT and would then pay them a retainer to provide information.'
He said it would cost from pounds 5 to pounds 25 to obtain an ex-directory number and that a copy of a telephone bill - which has details of all the numbers someone has rung - cost from pounds 25 to pounds 150.
Detectives meet BT employees by waiting outside their offices or getting them to visit their workplace or home on official business. 'They would start chatting with them and make them an offer.' He said the problem was the worst it had been in the 22 years he has been a detective.
'We have been telling the Government about this problem for years but they have done nothing.'
Other methods of obtaining information, none of which Mr Brogan use, include ringing up special BT helplines and pretending to be a police office or an engineer to obtain a confidential number.
There are an estimated 15,000 private investigators in Britain, but only 600 or so belong to regulatory bodies, which have ethical codes of conduct.
A BT spokesman refused to comment on the arrests or on the suggestions that its security was lax and that some of its workers sold information to private detectives.Reuse content