BT unveiled a deal today that will turn the wi-fi systems of its home broadband customers into part of a massive global network and allow the telecoms group to offer “free wi-fi” access at sites across the globe.

The former telecoms monopoly has taken a “substantial stake” in FON, the Spanish-based internet wireless group.

FON, which is privately owned, also counts Google and Skype as investors.

BT declined to disclose the size or price of the stake but did say that it would take a seat on the FON board.

The tie-up means that more than three million BT Total Broadband customers will be given the option of joining the FON wi-fi network, which provides more than 190,000 hotspots worldwide.

By opting to join the scheme, BT customers effectively will allow their home wi-fi equipment to be used as part of FON’s public network, which others member can then access through their home equipment.

BT FON said that it aimed to build a huge "community wi-fi network", covering hundreds of thousands of hotspots under the slogan "Wi-fi for everyone, I'm in".

The two groups said in a statement: “The revolutionary idea for a massive 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 of the home or office.”

Gavin Patterson, managing director of BT’s Consumer division, said: "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.

"We are giving our millions of broadband customers a choice and an opportunity.

"If they are prepared to securely share a little of their broadband, they can share the broadband connections of 190,000 others without paying a penny.

Martin Varsavsky, the FON founder and chief executive, said: “From the beginning FON users believed in the concept of sharing and in the peoples' ability to participate in building something important that would benefit everyone.”

Wi-fi gives consumers broadband on their laptops at home, or on the move, without a cable.

Mobile phone handsets are also being produced with wi-fi chips that will enable users to bypass their 3G mobile network and make cheap calls over the internet.

The technology has alarmed many mobile operators, which regard it as a threat to their revenues.

After demerging its O2 mobile operation, BT does not have a mobile network of its own — a factor regarded as a key weak spot by many analysts.

Alhough it runs a service over Vodafone’s network, it has only 304,000 mobile connections — down from 372,00 in 2004.

Technology such as wi-fi and wi-max — wi-fi’s more advanced rival that works over a broader area and at faster speeds — could help BT to overcome that problem, analysts believe.

Initally, wi-fi services were mainly available in busy locations, such as coffee shops and airports. Providers include T-Mobile and The Cloud.

However, entire areas are now being covered.

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