David Cameron is the new chairman of the National Citizen Service (NCS).
The NCS was set up by Mr Cameron in 2012. The service provides outdoor activities for 15 to 17-year-olds. The teenagers also work in the community.
The former Witney MP said the NCS was one of his proudest achievements during his six years as prime minister, and pledged to make non-military national service a "rite of passage" for every British teenager.
Mr Cameron said the NCS is "building bridges across social divides", and is an example of the "Big Society" in action.
"From the pilot projects that I began as leader of the opposition, to the full-scale programme that we have today, more than 275,000 people have taken part in what has become the fastest-growing youth movement of its kind in the world."