Sir Stephen Bubb, head of the chief executives body Acevo, has warned the Prime Minister, David Cameron, that the big society is "effectively dead".
In a letter to Cameron, Bubb says that while the government can take credit for "some major achievements" since 2010, including the establishment of the social lender Big Society Capital, and reforms to inheritance tax relief and Gift Aid, "elsewhere progress has been slowed or has stalled".
"The big society is a strong concept, even if Whitehall has struggled to communicate it or implement it consistently – but now the phrase is effectively dead," he writes.
"The potential for charities to transform public services remains largely untapped, with reforms in too many areas either glacially slow (as in the reform of social care funding or offender rehabilitation), or with the reality divorced from the rhetoric (as with the Work Programme, where too many charities are now struggling or withdrawing from what was first described as a ‘massive boost for the big society’)."
Bubb says many charities face "crippling spending cuts" and most Acevo members whose funding is being cut have had to reduce the services they provide. Despite the size and strength of the sector, its role in creating jobs and economic growth is generally ignored, he says.
"It is hard for those charity leaders to escape the feeling that when it comes to promoting the potential of charities in this country, a government once sincerely full of ambition, vision and urgency has lost its way, and now lacks a clear narrative on the role of charities in the economy and society," he writes.
Bubb says charity leaders would like to be "constructive, rather than fractious, partners" and hopes they can get the relationship between the sector and the government "back on track".
Cameron and the Deputy Prime Minister, Nick Clegg, are today expected to mark the half-way point in coalition government by unveiling new initiatives.
In the letter, Bubb urges Cameron to ensure that the coalition update does four things, including reaffirming the government’s commitment to supporting charitable activity. To do this, he suggests the government could help grow the number of employees donating to good causes through payroll giving by: building on the government matching scheme of the early 2000s; or committing to a "swift payback of the hundreds of millions borrowed by the last administration from the Big Lottery Fund to pay for the Olympics"; or committing to "substantial action to incentivise social investment".
The other three things the government should do to help the sector, Bubb writes, are: show ambition on reforming public services; reaffirm the government's commitment to protecting the most vulnerable; and give clear incentives for charities to create jobs and econonic growth as part of the broader initatives to promote the growth of small and medium enterprises.
A Cabinet Office spokeswoman said that the coalition government had been working from day one to help charities do more good by cutting red tape and introducing new incentives for giving and delivering Big Society Capital, and that it would build on the success of the Games Makers at the Olympics through its Join In and National Citizen Service programmes.
She said new opportunities were opening up for charities to help deliver public services, such as the Any Qualified Provider scheme in public health. The new Community Right to Challenge means voluntary groups can challenge local authorities to open up service delivery to competition, she added, while "the new Commissioning Academy is helping senior public sector staff to design and commission public services in a way that recognises the value civil society providers can bring".