YAHOO! and Microsoft are making their instant messaging (IM) systems 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 instant messages across their respective IM systems and eventually to make voice calls using VoIP (voice over internet protocol) technology. The joint platform, launched as a test – or beta – version today, 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.

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

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

The two companies claim to be attracting strong advertising interest and say that they are seeing significant growth in IM take-up. Yahoo! also charges users who use the service to make voice calls to conventional landlines.

Nielsen/NetRatings, the internet research company, says that 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 past year.

Philipa 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 that Yahoo! offers already.

The deal will also leave Google, widely seen as the arch rival of both Yahoo! and Microsoft, 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 that it would not rule out forging further similar partnerships.

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