Facebook is expected to launch a new messaging system seen as a challenge to Google's Gmail, Yahoo and Hotmail.

The giant social network which boasts of more than 500 million users worldwide will host a special event in San Francisco on Monday.

Unconfirmed reports suggest Facebook could announce its own e-mail address @facebook.com - or even a fully-fledged web-based e-mail system.

Google-Facebook rivalry has increased recently amid a series of disputes.

Google has banned Facebook from importing its users' e-mail contacts, a move, it says, it made because Facebook refused to allow the export of contact and friend data from within user profiles.

In reply, Facebook enabled a workaround that let users save their Google contacts to their computers before importing them into Facebook.

According to a report on technology blog Techcrunch, Facebook's new venture codenamed Project Titan is being dubbed a "Gmail killer".