Thousands of Indigo.ie email account holders faced lengthy delays to their email services yesterday following the spread of a computer worm called Swen.

The worm, which by last night had infected 1.5 million computers worldwide, masquerades as a legitimate email from Microsoft promising a software patch.

But the email automatically executes an attachment to infect a computer users' machine and also attempts to steal email account data.

The volume of emails prompted by the worm slowed some indigo account holder's email service yesterday.

Eircom, which operates the popular indigo.ie email service, confirmed that up to 5,000 of its account holders had received emails containing the virus.

Some indigo users experienced delays of up to an hour or more yesterday when they tried to send e-mails.

Eircom said it had moved to block any emails containing the worm from its indigo system but was dealing with a backlog of emails that should be cleared by late last night.

The appearance of the Swen worm in the Republic is the latest in a series of viruses and worms that has infected computers.

Experts have warned users never to open unsolicited emails and Microsoft has said it never sends software patches via email. Rather, it directs users to its website to download patches.