Several large public service contractors in the private sector are guilty of "poor treatment of civil society subcontractors", Sir Stephen Bubb, head of the charity chief executives body Acevo, has told the Prime Minister.
In a letter to David Cameron, Bubb says that his broader aims as set out in the 2011 white paper Open Public Services have not been achieved.
"The public sector monopolies that you were so keen to break open have been threatened by new private sector oligopolies," the letter says. Reforms were too focused on cost savings, it says, instead of on "engagement, collaboration and social value".
Bubb says charities working with the private sector were often "set up to fail", in particular those working in the Work Programme.
Contractors were "failing to refer sufficient clients and refusing to lessen the degree of financial risk to which subcontractors are exposed", the letter says.
The private sector firms have "been highly aggressive in their tactics and have cornered the market through their purchasing power", it says.
But Bubb says he still supports aims set out in Open Public Services, which he calls "as relevant and important today as in 2011".
Bubb’s letter calls for a full, Whitehall-led review into how to achieve "a renewed focus on citizens’ rights in our public services".
He suggests these "give serious weight to establishing two basic rights for citizens and communities within our public services": namely a "right to voice" and a "right to choice".
Speaking at the time of the launch of the open public services agenda in 2011, Bubb said the "any willing provider" model would provide "potentially huge opportunities" for voluntary organisations.
A spokeswoman for the Cabinet Office said: "We have opened up public sector contracts to the charity sector to an unprecedented degree, from health services to probation. We have also put in place a comprehensive package of support to help the sector make the most of those opportunities, investing in their commercial capacity and supporting their to access finance."