FIRMS and individuals with a permanent modem connection have been warned that they can run up huge telephone bills if they use the pay-by-minute Internet dial-up system.

Canterbury-based information technology firm ADM Computing has warned that a software glitch could leave machines on line.

The company says that one client's typical quarterly bill of around £200 soared to £2,800 after the firm unwittingly became connected for 96 hours. Its Internet service provider (ISP) failed to alert the firm to the problem and denied any responsibility.

Steve Bennett, ADM Computing's senior network engineer, said continuous line connection was caused by "a glitch in the software". It was due to the way an anti-virus protection programme worked rather than a major fault.

The software was widely used and the client's problem of permanent line connection was not an isolated case. Linda Sinclair, ADM Computing's customer service director, said it was important for firms to be aware of the risk of using a dial-up connection.

It was far better to adopt the ADSL or broadband system, or pay a fixed monthly fee. Ms Sinclair said: "We are now recommending to our customers that they seriously consider switching from "pay by minute" dial-up Internet connections to broadband.

"Broadband connections, such as ADSL, are permanently open at a fixed cost per month, therefore eliminating the danger of extremely unpleasant and expensive surprises when your telephone bill arrives."