Users of Windows Live Messenger can now report suspected sexual predators of children with a mouse click.
A "report abuse" icon will soon appear on the chat software as a result of work by the UK"s Child Exploitation and Online Protection Centre (CEOP).
Users will be encouraged to click the icon when they suffer or witness inappropriate sexual contact.
CEOP said, if necessary, reports would be passed to police forces around the world to track down sexual predators.
"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.