BT currently boasts more than three million Total Broadband customers, all of whom will be invited to join the community of people sharing their home broadband connection.
Every person who agrees to share a small portion of their connection by opening a separate secure channel on their wireless router will be able to share the connection of any other member.
BT Total Broadband customers will also be able to use BT's existing Openzone hotspot network.
The idea is to create ubiquitous Wi-Fi coverage built by individual people and not a large corporate enterprise.
"This is the start of something very exciting for BT. Today we are launching a people's network of Wi-Fi, which could one day cover every street in Britain, " said Gavin Patterson, group managing director of BT's consumer division.
"We are giving our millions of Total Broadband customers a choice and an opportunity if they are prepared to securely share a little of their broadband.
"They can share the broadband at hundreds of thousands of FON and BT Openzone hotspots today without paying a penny."
New members will become part of an existing community of 500,000 people and will have free access to more than 190,000 FON hotspots worldwide plus the new BT FON hotspots.
BT FON aims to build a huge community Wi-Fi network covering hundreds of thousands of hotspots in a short space of time.
"From the beginning FON users believed in the concept of sharing and in the people's ability to participate in building something important that would benefit everyone," said Martin Varsavsky, chief executive and founder of FON.
The secure open Wi-Fi platform was developed at FON's and BT's research labs. BT has invested in FON as part of the tie-in, joining the company's other investors which include Google. BT will also have a seat on the board of FON.
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