David Cameron is proposing a voluntary "citizen service" programme for 16-year-olds, which he sees as a 21st Century version of National Service.
The Tory leader said school leavers and those going to college should take part in a six-week programme, including charity work and physical training.
He said it would boost participants' pride in themselves and in Britain.
But Cabinet Office Minister Ed Miliband said the Tory proposals were "neither costed nor funded".
Mr Cameron unveiled his plans for a modern equivalent of national service - which would not be compulsory - during a visit to Bolton, alongside British boxer Amir Khan.
'Transitional age'
He said he had initially favoured a compulsory scheme, but had been persuaded against that and to make it "universal" instead by the voluntary sector.
However, he said he would consider it a failure if all 16-year-olds did not eventually take part.
"I'm absolutely determined to make this work. In my view this is something so important for the future of our society ... we can't afford not to make it work," he said.
Mr Cameron said 16 was the right age to take part, when "people are thinking about that transition from youth to adulthood".
"I want all Conservatives to think carefully before they open their mouths"
Earlier he told the BBC youngsters from all backgrounds - "north or south, rich or poor, black or white" would take part together and it should be "challenging and exciting".
The programme was aimed at taking youngsters out of their "comfort zones", whether they wanted to go on to further education or straight to work, he added.
Physical challenge
The course would include one week's residential course, four weeks of community service and a week's physical challenge - like Army training or mountain climbing.
It could include work with the elderly or travelling to developing countries.
Mr Cameron suggested participants would be eligible for a cash sum on completion of their service, with half going to a charity of their choice and half to the organisation which ran the project.
"The important thing is that all parties agree that young people get opportunities, young people get the chance to do community service. Our 'V' programme is already doing that"
He said the cost of the scheme had not yet been worked out, but it would ultimately save money by cutting crime and anti-social behaviour.
Labour is in favour of more recognition of, and a greater role for, the voluntary sector and Gordon Brown has proposed a day to celebrate the British national identity.
Downing Street also said Mr Brown had launched a scheme to encourage teenagers to volunteer, called V, in May 2006.
Asked about the proposal, Mr Brown said: "I think the important thing is that all parties agree that young people get opportunities, young people get the chance to do community service. Our V programme is already doing that."
But he said it was "only part of the issue" and must be undertaken alongside school discipline and leadership.
Mr Miliband said: "Residential opportunities are important for young people but the problem with David Cameron's proposals is that they are neither costed nor funded.
"All the evidence is that this would be hugely expensive and he hasn't a clue how he will pay for it."
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