Cameron to 'bring Britain together.
He said 11 organisations, ranging from the Duke of Edinburgh Award scheme and the Prince's Trust to the National Union of Teachers, had agreed to meet him to discuss details of his school-leaver programme.
They would debate whether the scheme should be compulsory - or if not, how to ensure the widest take-up - and what sort of approaches would work best.
Mr Cameron told a Politics Studies Association conference on British values in London: "Those voices who are warning us about the dangers of ghettoisation in our country and a disintegrating sense of national cohesion are absolutely right.
"We need to bring people together and bring Britain together. I think that the best way of bringing people together is to enable them to do things together, to build something together that is of lasting value.
"I am always struck when asking anyone of my father's generation who did national service by the fact that they tend to reply in a similar way.
"It was something we all did together - irrespective of who we were, where we lived, where we came from, or what god we worshipped.
"Today, university is our closest equivalent, with each campus becoming a melting pot mixing together all the elements of our country. But can that ever be enough?
"Isn't there more we can do to enable young people to come together and give service to their country?"
He said he wanted to challenge the organisations and the armed forces to develop ideas for a school-leaver programme.
"Something that prepares teenagers for their responsibilities as adult citizens - that enables them to meet people from different backgrounds, and to learn about the realities of life in different communities, and which teaches them the lifelong lesson that we're all in this together.
"This idea, and this approach, goes to the heart of my political philosophy. There's not a single challenge that's not best tackled by asking what can we all do about it - government, individuals, families, businesses, voluntary organisations.
"And that's the right approach to politics - not waiting around for the Government to do things, but bringing people together to make things happen.
"I believe that creating a national school-leaver programme is exactly the kind of positive, optimistic change we need to make a tangible reality of the important discussions on British values that you're having today