Residents of Muswell Hill and Whitchurch will be the first to get next generation broadband.
Exchanges in the North London and South Glamorgan areas will be the first pilot sites for the fibre-based broadband and once upgraded, home users should be able to go online at speeds of up to 40 megabits per second (Mbps).
Around 15,000 home and business customers in each area will get the opportunity to take part in the pilot which will run fibre-optic cables to the street cabinets that connect homes and businesses to telephone exchanges.
The pilots will begin in summer 2009. Prior to the start of the large scale trials BT's Openreach will run a small trial in early 2009 involving 30 homes connected to the Foxhall exchange in Kesgrave, Suffolk.
A BT spokesman said the two sites were chosen because of their mix of customers and net providers in these areas as well as the make-up of the local communications network and geography.