David Cameron's flagship national service scheme for young people needs "radical thinking" to justify its continued operation, a parliamentary report has warned.
The House of Commons Public Accounts Committee (PAC) found that the scheme, which has received £475 million in taxpayers' money since 2014/15, may no longer be justifiable unless it can meet "ambitious" targets for increasing numbers of participants and proving its value to society at a lower cost per head.
In 2016, NCS courses are expected to cost taxpayers £1,863 for each participant, the report found.
It was "not at all clear" why this was so much higher than the £550 cost of creating a four-year place for a young person in the Scouts.
The former prime minister set up National Citizen Service as part of his "big society" programme, with the aim that it would become a rite of passage for teenagers as they prepare to enter adulthood.
After leaving office last year, he took up a post as chairman of its panel of patrons.
Since 2011, more than 300,000 16 and 17-year-olds have taken part in NCS, usually on four-week summer programmes involving residential courses and community projects.
Office for Civil Society evaluations suggest that the schemes have a real impact on those taking part, said the report.
But the PAC said that the NCS Trust and Department for Culture, Media and Sport could not justify the seemingly high cost of courses.
And it said it was unclear whether the Trust's management had the necessary skills and experience to oversee the programme's planned growth.
The committee voiced disappointment at the Trust's "relaxed attitude" towards its failure to recover £10 million from course providers for places that were not filled in 2016.
And it said it was not clear why the Trust was reluctant to disclose information including the salaries of its directors - though the committee accepted that it was not required to do so.
PAC chairwoman Meg Hillier said NCS had reached a critical juncture.
"The Government intends to push on with plans to grow participation, citing evidence that NCS has had a positive impact on young people who have taken part," said the Labour MP.
"However, this does not in itself justify the level of public spending on the programme, nor demonstrate that NCS in its current form will deliver the proposed benefits to wider society."
Ms Hillier said that 99% of the Trust's income since 2014/15 had come from the public purse.
"This and future commitments are significant sums yet it is not at all clear why NCS participation costs should be so much higher than those for a voluntary sector organisation such as the Scouts," she said.
"Nor is it clear why the Trust, as the recipient of this public money, should apparently be so reluctant to voluntarily disclose financial information such as the salaries of directors.
"This attitude does nothing to build public confidence in an organisation that has lacked discipline in recovering overpayments of taxpayers' money, while running a programme for which there is still no clear evaluation plan."
NCS Trust chief executive Michael Lynas said the organisation was considering the committee's report with the aim of delivering on its recommendations and making citizen service "a normal part of growing up for young people in our country".
"Achieving value for taxpayers' money is of central concern to NCS Trust and independent research shows between £2.20 and £4.15 of benefits are returned to society for every £1 invested," said Mr Lynas.
"The vast majority of NCS funding goes to more than 200 local charities and community groups and helps them to build the local infrastructure to attract young people to NCS and deliver life-changing programmes.
"We conduct regular audits to ensure that this funding is being used in an optimal way and have remained within our budget set by Government in every year. As the programme grows, we are recommissioning our network, allowing us to take advantage of scale efficiencies and new ways of delivery to lower our unit cost, as the report recommends."
A DCMS spokesman said: "NCS has had a positive impact on the lives of over 300,000 young people to date and we want many more to benefit from it.
"Independent evaluation has demonstrated that NCS is delivering value for money and we will be looking closely at how we can better measure the impact it has in the long term. The NCS Bill, in its final stages in the House of Commons, will also ensure that the NCS Trust is directly accountable to Parliament for its use of public money."