The swarm-like energy that drives social networking sites like Facebook is to be harnessed in an attempt to expand the reach of the nation's wi-fi network.

From this week, home owners will be invited to share their home broadband access with passers-by in return for the right to access the network through others' connections when they are roaming with an internet-ready device.

The collection of shared wi-fi hotspots aims to provide a nationwide corridor of internet access, not just to laptops but to any internet-enabled device. These days, that includes many high-end mobile handsets and PDAs and, from early next month, Apple's iPhone.

Under a deal announced between BT and FON, a Spanish wireless internet company, people who sign up will be able to access the internet free through others' home broadband connections, as well as through BT's 2,000 wi-fi hotspots across the country.

Home broadband routers have a range of about 30m, and cast a 'shadow' of connectivity over nearly 3,000 square metres, making it easy for passers-by to 'piggyback' on the signal from outside.

For existing BT broadband subscribers, access will be free. Those with other providers need to pay a one-off £30 fee for a FON router and register with its network, at which point they can log on anywhere with a user name and password.

There are now more than 10,000 wi-fi hotspots across the UK - the majority being in airports, railway stations, and coffee shops - but almost all are paid-for services, starting at about £6 an hour or £10 for a day's access.

BT and FON's thinking is that, as more and more phones become wi-fi enabled, there will be a demand for free, broadband-speed internet access on such devices. Accessing the web via a mobile phone network can be painfully slow.

The number of phones across Europe which have wi-fi capability - currently about 3 per cent - is expected to grow to more than 40 per cent in the next three years, according to the analyst company Gartner.

FON would not say how many UK residents were already part of its network, but a spokesman said that as soon as the number reached 50,000, the service would start to be useful.

"The revolutionary idea for a masssive wi-fi community, built by individual people and not a large corporate enterprise marks BT's boldest step yet in building extensive broadband coverage outside the home or office," the company said in a joint statement with BT.

This week McDonalds also announced it would be rolling free wi-fi out across its 1,200 restaurants by the end of the year.

Members of the BT/FON network will only 'give up' 512Mb of their home connection, FON said - enough for moderately fast broadband access, though in hotspots with greater demand, like major train stations, speeds would be greater.

Sceptics said that it was unlikely people would be in areas of shared connectivity - predominantly residential streets - when they wanted to use free wi-fi, and that quality control would be difficult to maintain.

FON said that in other countries the majority of its customers were in urban areas, meaning that there was coverage "in the places that matter," and that the inherent problems of any wi-fi signal - pockets of poor connectivity, and occasional interference - wouldn't be any greater in its network.

Security experts also warned that there could be "significant risks" for consumers in opening up their home broadband connections, pointing out the many instances already where a failure to attach a password has led to users unwittingly sharing their network.

"My first thought was: wild horses wouldn't get me to sign up to this," Paul Vlissidis, technical director of NCC, a security firm, said. "It's not just the risk of having personal details taken. What about if someone starts downloading child pornography via my connection? Am I implicated?"

Diego Cabezudo, FON's chief operating officer, said that the public and private parts of a home connection were kept "completely separate" and that no 'traffic' from outside the home was allowed to access the owner's private network.

The source of any "malicious activity" could also be identified, and authorities notified where necessary, he said.

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