The short-haired bumblebee vanished from the UK in the 1980s, after 60 years of declines as its wildflower-rich habitat was lost. It was declared extinct in 2000.
A project has brought queen bumblebees over from Sweden and after two releases of queens at the RSPB's Dungeness site in Kent, offspring worker bees have been recorded there.
Nikki Gammans, who leads the project, said: "This is a milestone for the project and a real victory for conservation. We now have proof that this bumblebee has nested and hatched young, and we hope it is on the way to become a self-supporting wild species in the UK.
"It's been a long journey to get here, from creating the right habitat for them, collecting queens in the Swedish countryside, scanning them for diseases and then eventually releasing them at Dungeness."