Several Irish companies and organisations were forced to shut down their computer systems this week following infection by the fast-spreading "Nimda" worm.

Assurer Friends First and telecoms group Ericsson were forced to shut down their systems on Wednesday while, in an ironic twist, the worm also forced the telecommunications regulator's website offline for most of Wednesday and Thursday. The Irish Times was also affected by Nimda.

An Ericsson spokesman confirmed yesterday that some of its worldwide systems were hit by the worm and had to be shut down. This had not impacted on business activities, he added. But market researcher Computer Economics said on Wednesday that the global damage caused by Nimda stood at about $530 million (€571.5 million) and could rise.

"This is one of the worst we have heard of. It has affected a number of high-profile companies in the US and hit them badly," said Mr Dan Quealy, director of Ernst & Young Security & Technology Solutions Practice Ireland.

Nimda is a versatile program that spreads itself by e-mail and the Web, slowing down computer systems. The e-mails carry an attachment entitled readme.exe, but can infect without this being opened. Accessing infected websites can transfer the virus.

Nimda is also targeting PCs, which has allowed it to spread faster and made it harder to track.