The move will allow BT's broadband customers and BT Openzone users to email, surf the internet or make VoIP calls at Starbucks as part of their bundles.
O2 iPhone users will have access included within their contract, and customers from iPass, Boingo and other BT Openzone roaming partners will also be able to use the service.
The first stores will have the service installed this week and the roll out will be completed by the end of the summer 2009.
Chris Bruce, chief executive of BT Openzone, said: "Starbucks is the perfect place to work or catch up over email.
"Use of our WiFi doubled last year and coffee shops are increasingly popular as a third place to work or log on to the internet.
"We're constantly looking to boost the choice of places to connect -- where people want it.
"The location of Starbucks stores in the high street and travel hubs is helping to extend the reach of WiFi to customers."
Starbucks will use the network to support its store operations systems.
Robert Teagle, Starbucks EMEA IT director, said: "At Starbucks, we are continually looking for new ways to develop our in-store offerings for our customers, and this new relationship with BT, provides an enhanced customer experience.
"As we continue to develop our technology strategies this relationship will strengthen the Starbucks Experience and amplify our stores as a must-go location for our customers."
An existing BT Openzone roaming agreement with AT&T also allows users access at more than 7,000 Starbucks stores in the US.
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