Charity leaders have accused David Cameron of neglecting his Big Society idea.
Senior figures said the concept - a central plank of the Prime Minister's 2010 General Election campaign - appeared to be "going nowhere".
The message came in a letter penned by Sir Stephen Bubb, head of the Association of Chief Executives of Voluntary Organisations, which represents around 2,000 charities.
"As Prime Minister, you described building a Big Society as your 'great passion' and 'central to my vision for our country'," he wrote.
"You spoke eloquently of your desire to reform public services, with a significantly greater role for charities."
However, Sir Stephen said the potential for charities to transform public services remained "largely untapped, with reforms in too many areas glacially slow".
"The mood music across Whitehall has been that reform is off the agenda. The reality many charities now face is crippling spending cuts," he went on.
Speaking on the BBC Radio 4 Today programme, Sir Stephen insisted the argument was not about the spending cuts, but about "delivering public services better".
"David Cameron's vision of the Big Society is something we want and we want to help him achieve it," he said.
"There is huge frustration amongst charity leaders that these ideas about reforming public services don't seem to be going anywhere."