Yahoo! and Microsoft are making their instant messaging (IM) systems inter-compatible to create a communications platform with some 350 million user accounts.

The alliance, the first of its kind, will allow Yahoo! and Microsoft users to send text messages over the internet and eventually to make voice calls using VoIP (voice over internet protocol) technology through PCs. The joint platform, launched as a test – or beta – version today (Thursday), will eclipse the 100 million user base of Skype, the internet telephony company acquired by eBay last year in a deal worth up to $4.1 billion (£2.2bn).

The partnership will also up the pressure on telecoms incumbents such as BT, which has around 19 million user accounts. Traditional telecoms groups are seeing margins eaten away as users migrate to VoIP systems where calls are often virtually free.

Glen Drury, managing director of Yahoo UK & Ireland, told Times Online: "This idea has been around there since the early days of the internet and has consistently topped the user-request charts. Now that the security issues have been tackled and we are able to bring together what are two very complex bespoke systems."

No cash will be exchanged under the terms of the partnership and Microsoft and Yahoo! will continue to compete for users.

Both Microsoft’s Windows Live Messenger, which has more than 240 million active registered users, and Yahoo! Messenger, which has around 110 million accounts, will also compete for advertising revenue – both IM system’s main source of income.

The two companies claim to have benefited as advertisers are attracted to the young, technology savvy users of IM services. Both say they are seeing "significant" growth in IM take up. Yahoo! also charges users who use the service to make voice calls to conventional landlines

According to Nielsen/NetRatings, the internet research company, 14.1 million people in the UK, around half the active internet population, used an IM in May. The number of UK people using an IM has grown by 22 per cent over the last year.

Phillipa Snare, head of Microsoft’s Communications Services division, said: "Instant messaging services are becoming a communications method of choice. There is a change taking place around the technology similar to that mobile phones underwent in recent years."

To extend the technologies reach, Microsoft and Yahoo are currently looking at moving IM services to mobile phones – a service which already Yahoo offers.

The deal will also leave Google, widely seen as the arch-rival of both Yahoo! and Google, out in the cold. Google launched Google Talk last year in an effort to break the VoIP market but user numbers have so far been disappointing.

Yahoo!, which recently announced a commercial partnership with online auction house eBay, said it would not rule out forging further similar partnerships.

"When you’re working on this scale, working on one deal at a time is enough to keep you busy but who’s to say what will happen next?"

Times Recommends

• Food prices rise 8.3% on soaring fish and meat
• Car sales crash as economy hits skids
• House prices fall but Bank leaves rates on hold