The problem affected a company which processes messages delivered through the Virgin.net platform.

All legitimate e-mails were held back when the "large" attack began on Tuesday evening so the spam could be removed, a Virgin Media spokesman said.

E-mails were gradually being delivered now the fault was cleared, he added.

He said these messages should now have been available but access via webmail may have been taking longer to restore.

'Nothing lost'

About 200,000 people use the Virgin.net service but well under half of these were affected by the problem, the company stated.

Error messages were displayed when they tried to log on. Virgin Media stressed that no e-mails had been lost.

"Since late Tuesday evening, some customers on our Virgin Media DSL service may have been unable to access e-mail or webmail," the spokesman said.

"This was due to a suspected spam attack suffered by our e-mail supplier which also affected a number of other ISPs. No customers on our cable service were affected.

"All customers are able to access e-mail again now, though due to a large queue of undelivered e-mail messages, there may be a short delay before some e-mails reach customers' accounts."

BBC