"Legal action against such lists was always on the cards, instigated either by legitimate e-mail users or spammers."

He said that if blacklists were closed down ISPs would likely continue to use existing blacklist information, but its effectiveness would diminish quickly as new spammers came online.

"The resulting flood of spam would leave business people ploughing through a host of unwanted e-mails for every legitimate message received, which would curtail commerce significantly," Stewart said.

"Crime would rise sharply too, as e-mails bearing phishing scams, spyware, keyloggers and other malware arrived unchecked in millions of in-boxes. Overall the closure of blacklists would have a dramatic effect on global business efficiency."

Stewart likened the effect to police stations going on strike, leaving skeleton staff to try to cope, unsuccessfully, as the criminal element gained in strength and confidence.