Antivirus software developed by scientists at Dera, the government's Defence Evaluation and Research Agency, will be freely available to all computer users from today.

The SyBard/Mail software, which can be downloaded for free, is designed to stop viruses spreading to other users via email address books. This is the way the Kournikova virus spread earlier this year.

"By making the software freely available for home computer users, we are making a major contribution to the fight against the malicious email threat," said Dr Simon Wiseman, the leader of the research team at Dera that developed the software.

The software works by creating a dialog box that double-checks with the user that the email is to be sent. It resembles boxes found in Windows.

It immediately notifies the user of all emails that have not been generated by the dispatch of a genuine message. The user can then delete the email and prevent the virus being spread to other computers.

But as with most antivirus software its effectiveness is reliant on the vigilance of its user.

"Any system is reliant on the user being aware of the dangers [of viruses]," said Dera spokesman Jonathan Byrne. "It is up to the user to be careful and this helps them to be."

Some experts feel the software may be time consuming, perhaps even intrusive, forcing the user to verify the sending of every email.

"Why not stop the virus getting in your computer in the first place?" asks MessageLabs' antivirus specialist Alex Shipp.

Dera's Dr Wiseman countered this by saying the software is not a replacement for detection software but stops the spread of viruses. It also buys time for people trying to deal with viruses, including antivirus experts such as Shipp.

"It is one additional click," said Byrne. "People have accepted this same functionality within Windows." It is a long-established truism that computer security systems should be used in tandem, rather than on their own.

The Sybard/Mail Lite software is part of an integrated computer security solution under development. Other parts of the system will be released in stages over the coming year.

"The software is secure and in tests it has proved extremely resistant," said Byrne. "It would not let any rogue applications activate the send button, so it is very effective."