Almost unnoticed, it has become a criminal offence to attach unapproved equipment to the public telecommunications network.

British Telecom last week denied rumours that it was planning a crackdown on people using unapproved modems and other telecommunications equipment.

But fears of tougher enforcement remain, following key sections of the Telecommunications Act coming into force last month.

Under the Act it is, for the first time, a criminal offence to attach unapproved equipment. Previously British Telecom relied on the threat of disconnection to deal with offenders or, in extreme cases, it took people to court for stealing its electricity.

Now a criminal prosecution can be brought by a local Trading Standards Officer, BT or the Department of Trade and Industry. Anyone found guilty will be liable to a fine of up to £2,000.

It is relatively easy for BT to find out if you're using unauthorised equipment like an unapproved modem. The line testing equipment is uses is sensitive enough to detect whether a subscriber has more than just a single phone attached to the line.

A quick check of its records would reveal whether the subscriber has had the additional equipment installed by BT.

Its suspicions aroused, BT could send an engineer around to your house to check your phone.

The extent to which BT will use this procedure to launch a deliberate clamp-down remains to be seen though some people think it unlikely. "Our engineers have got better things to do," said one BT source.

But, with the rapid increase in the use of modems by micro users and the lengthy approval procedures of the British Approval Board for Telecommunications (issue 52), BT could decide that the numbers of unapproved modems in use warrants action.