Microsoft and Yahoo are preparing to link together their free instant messaging services as they take on entrenched messaging leader AOL and market newcomer Google, a source close to the companies said on Tuesday according to the news agency Reuters.

If the deal was to go ahead, it would allow users of both systems to talk to each other via the two programs and be the first major alliance between two of the web's main providers of instant messaging.

The story first suggested in the Wall Street Journal implies that an announcement will be made later today to give both services the ability to communicate via voice as well, a feature that up to now has been restricted to users within each service, the source said.

Microsoft has already opened up its corporate online messaging service, which requires a license and offers more features, to AOL and Yahoo.

Being able to send instant messages to different services using a single program isn't new, however.

Users of unified messaging services such as Trillian can use a single software program to send and receive instant messages from AOL, MSN, Yahoo and other providers, as long as they have an account with each service.