Global investigation

"What Microsoft and the CEOP are doing today is saying is 'enough is enough'," said Jim Gamble, head of CEOP in a statement. "By working together in a very clear and tangible way we can safeguard children from online sexual predators."

Those clicking on the icon will be given advice about how best to capture evidence and take copies of online discussions.

Instant messaging programs such as MSN Messenger are hugely popular and give people a way to swap text chat with friends they know are online. Microsoft said it had "millions" of MSN Messenger users in the UK.

Because the reporting is not anonymous CEOP hopes to filter out any nuisance or malicious reports of abuse.

The tab will be seen on Windows Messenger and its replacement Windows Live Messenger.

CEOP is backed by the government and was set up in April 2006 to act as a single point of contact for organisations combating or investigating the abuse of children on the net.

CEOP is a member of the Virtual Global Taskforce (VGT), a global association of law enforcement organisations and anti-abuse organisations that try to combat online sexual abuse and grooming. CEOP said it would work with VGT partners to investigate reports.