Ministers have begun a push to publicise the second year of the National Citizen Service (NCS), David Cameron's personal crusade to introduce a voluntary version of national service for 16 year-olds.

The project started last year, with just over 8,000 young people taking part in summer holiday camps across Britain, designed to foster a sense of nationhood and community spirit, particularly among kids from deprived areas and ethic minority backgrounds.

The government sees this as one of its major Big Society projects - and will pump about £350m into the summer camps in 2012, despite concerns that the scheme failed to attract enough people last year and fears that the project doesn't have a sustainable footing in years to come - when ministers hope up to 90,000 teenagers will eventually take part in the summer camps.

In an interview with HuffPost UK one of the government's key Big Society ministers, Nick Hurd, accepts that the funding model - which he hopes will be increasingly picked up by the private sector - is unproven. He insists that the NCS remains in a pilot stage, despite claims that the government is ploughing ahead with a project which lacks widespread public enthusiasm or a long-term sustainable plan.

He told Huffpost UK on Thursday afternoon: "We're still very much in pilot phase, refining the model, reflecting on it. We're comfortable that it's a justifiable investment by the taxpayer. As we get to the point where the numbers are towards that 90,000 and beyond, there is going to be a big issue of where the funding comes from, the balance of contributions."

But he says the scheme was highly popular last summer, telling us: "I've had emails from people, saying 'I've worked in the youth sector for 30 years and this is the best youth programme I've ever been involved with.' "

Hurd described the scheme as "very powerful and very important for us in terms of social cohesion."

This year up to 30,000 teenagers could find themselves on three-week placements across the country, which the government says will help them "gain valuable life experience and new skills, make friends from all walks of life and prepare themselves for their futures – while playing a part in making their communities better."

The cost to to the taxpayer will be about £1,000 per teenager, with the whole scheme expected to cost £350m. Critics of the project - including the Tory-led Commons education committee - point out that this spend alone almost exceeds the entire amount earmarked for the rest of youth services across Britain.

The drive for the expansion of the scheme comes straight from Number 10 - with David Cameron believing that the case for a form of national service was proven by last summer's riots in several English cities. The idea was given a big boost just before the 2010 general election when Michael Caine endorsed it, standing alongside Cameron at a news conference to plug the scheme.

It has plenty of critics within the sector. Michelle Wright from the charitable enterprise company Cause4 told HuffPost UK that there were strong arguments for spending the money on existing schemes, rather than pumping new money into a seperate one. "The persistent questions around the NCS remain whether or not the scheme provides value for money," she said.

"Last year Parliament recommended reducing the plan to a gold standard mark for existing voluntary schemes as a way of keeping the operating title, but reducing the cost significantly. This could then support the promotion of existing local volunteering schemes, removing some of the duplication of work currently made by the NCS."

Andy Mycock, from the politics department at the University of Huddersfield, has been a vocal critic of the scheme since launch, and offered this assessment to HuffPost UK on Thursday night:

"Many of the problems regarding suitability of NCS for young people from disadvantaged communities and its long-term legacy were identified in a Conservative-funded research project conducted by Strathclyde University in 2009. However the report was quietly buried and is now only available here.

"A number of questions need to be answered before the NCS programme should be expended further. Foremost is why the NCS pilot failed to meet its recruitment target of 11,000 or why so many young people failed to complete the programme.

"Why did the Cabinet Office instruct some providers to accept young people from ages of 15-18 rather than just 16 year-olds? Why were some provider organisations allowed to make a loss due to poor recruitment? Why were the implications of holding NCS during Ramadan (August 2011) not addressed prior to the pilot?"