Serious system failures during the past 10 days in MCI WorldCom's US-based data networks have crippled US company networks, caused automatic teller machine systems to collapse and slowed the Internet in an indication of the potential fragility of digital networks.

The company's failure to directly address the problem publicly has lead to widespread criticism in the US. The shutdown closed some of the operations of the Chicago Board of Trade for five days.

While Irish Internet service providers say they are tracking the problem, both Ireland On-Line and Esat Net said they had not experienced any significant Internet network slowdowns. But several Irish industry sources said Irish companies with parent offices in the US that use MCI WorldCom's data network services may have experienced problems. MCI WorldCom US spokeswomen, Ms Linda Loughlin said the company had been contacting "affected customers", many "by e-mail and letters", but declined to say whether Irish or other European customers were being notified. The problem began last Thursday week and is centred in MCI WorldCom's frame relay network, a network similar to the Internet that companies use for transferring large amounts of data securely from one specific site to another or within a company network.

Ms Loughlin said the problem was caused by a glitch in Lucent software used for a network upgrade. She said the company was working with Lucent to resolve the problem and that most companies would be back online by the weekend.

In the US, MCI WorldCom controls about a fifth of the $5 billion (€4.7 billion) frame relay market and said this week that 30 per cent of their frame relay customers had been affected by the problem.