BT has taken up one of the oldest ideas in Wifi to hold out the promise of global wireless internet access.

Way back in the days when homes first began to sprout Wifi routers it was suggested that these could have both a private and a public face, with access available both to the householder and passers-by.

The idea was taken up by the Spanish hotspot provider Fon, which claims to have a network of 500,000 access sharers, known as foneros, in Europe Asia and America.

Now any of BT's three million Total Broadband customers can opt into the scheme for free. In return for allowing others to use their line, they get free access to any fonero link anywhere.

Guest foneros are limited to 512Kbits/sec on a secure pipe that gives priority to the host's traffic. If a user's traffic is capped, any fonero use counts towards the base account so that bandwidth acts almost like currency - a transition some have also predicted.

The system does not provide roaming access, like a mobile phone. That is there is no 'handover' between access points so that you have to stay within range to maintain a link.

BT subscribers using the company's Home Hub will not need to change their router but Fon offers one tailored for the system. BT owns a slice of Fon and the system was developed in conjunction with BT's research labs.

Gavin Patterson, managing director of BT's donsumer group, said: "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."

Other Fon investors include Google, Skype and Sequoia Capital.

The Fon deal is exclusive to BT, so that other service providers including those reselling BT bandwidth will not be able to follow suit.

The relatively low available bandwidth - equivalent to first-generation ADSL - is way below that promised by cellular HSDPA data links.

Pipex Wireless is expected today to announced the first commercial rollout of yet another competitor, Wimax. It will be in Manchester, under a new Freedom brand, but details will not be revealed until this afternoon.

A BT spokesman said the timing of today's Fon announcement is a coincidence.