Spammers prefer Thursdays to hit the send button, according to research from BT Openworld.

The Internet Service Provider (ISP), which is rolling out an anti-junk mail service to its customers this month, found that most spam arrived in in-boxes on a Thursday.

A worrying factor is that one in 220 spam emails has some kind of virus attached, prompting BT Openworld to call on its rivals to take a more proactive stance in combating spam.

Duncan Ingram, the ISP's managing director, told Computeractive that ISPs needed to take measures to help stem spam by blocking as much as possible before it is sent on to customers' accounts.

Last November, BT Openworld introduced anti-spam and antivirus services as standard for customers taking its unmetered services. This month it will extend the services to its Openworld Pay-As-You-Go and Broadband Direct customers for £1 per month.

"Not enough ISPs are doing enough to stop spam," said Ingram. "We have introduced anti-spam measures for all our customers and found that, of 25 million emails we monitored, 41 per cent consisted of spam."

Ingram would not rule out the future option of UK ISPs taking more aggressive steps against spammers.

"Legal action tends to take a long time, but we do not rule out steps like this," he said. "The only problem is most of the spam originates outside the European Union's jurisdiction."

AOL, Microsoft and Yahoo are working together on how to identify suspicious email headers, give better feedback options for consumers and find effective ways of preventing spam being sent from open relays or open proxies.