The move is the latest phase of BT's ongoing strategy of expanding its ownership of the customer from landline phones to mobiles and then PCs.
It will become a key focus of BT's ongoing efforts to generate money out of broadband beyond access revenues.
Communicator is being developed by BT and Yahoo! and will form a key part of their strategic partnership to develop a premium broadband service.
It is expected to launch early next year and will go head to head with Microsoft's latest version of MSN Messenger.
Andrew Burke, director of online services for BT Retail, said: "Bringing all of these functions together on the PC will make it a much stronger communications device. We want to offer our customers the choice of using the PC, fixed line or mobile phone to communicate."
Communicator will be made available to both BT Yahoo! customers and BT's own customer base. Elements of the service, such as instant messaging and PC-based phone calls, are expected to be free, although BT is planning to charge for other parts.
BT and Yahoo! will market the service with a high-profile cross-media campaign expected to form the next phase of its marketing strategy for BT Yahoo! The first £10m work, through Clemmow Hornby Inge, featured two fictitious US characters from the web's early days (Marketing, September 4).