The energy supplier has now fitted more than 1.4 million smart meters in homes and in businesses, it said in a statement, with the one millionth British Gas smart meter installed in a home in Lewisham, south east London.
The mass rollout of smart meters is due to start next year, and all homes and small businesses are due to have smart meters fitted by the end of 2020.
Energy secretary Ed Davey said: “The fact that British Gas is installing its millionth smart meter in a home shows that helping people take control of their energy use and reduce their bills is becoming a reality.”
Sacha Deshmukh, chief executive of the Smart Meter Central Delivery Body said it was a “landmark moment”.
He added: “This provides a vital foundation for a transformation that will reach many millions more homes around the whole of Great Britain in the years ahead.”
Jorge Pikunic, managing director of British Gas Smart Metering said: “At British Gas we want our customers to be the most informed and best equipped for energy efficiency, so we’re working hard to ensure they receive the benefits of smart meters as early as possible.”