Nick Hurd, the former Minister for Civil Society, has urged the Prime Minister to do more to enable charities and social enterprises to win public service delivery contracts.
Hurd resigned from his post on Monday evening after more than four years in the role.
In his resignation letter to David Cameron, Hurd said he was grateful to have been able to spend four years managing a brief he cared passionately about.
"It has been a privilege to work with our extraordinary voluntary sector through a very challenging time," wrote Hurd. "They really do represent the best of British values."
But Hurd also urged Cameron to do more in some areas, including public service delivery by voluntary sector organisations.
"There is of course much more that can be done to make it easier for social entrepreneurs and socially responsible businesses to create social value," said Hurd.
"The social value act was a useful start but cannot be the end of our efforts to encourage intelligent commissioning from a diverse market of suppliers, including charities, public sector mutuals and social enterprises.
"We have yet to unlock the full value that they can add in helping us tackle some of the most stubborn social challenges that undermine the country. It is hard to see us meeting the challenge of delivering ‘better with less’ without our social economy fully engaged."
He also urged Cameron to push for a total of one million graduates from the National Citizen Service by the end of 2020 – a project Hurd described as "the most inspiring work I have ever done".
Hurd paid tribute to Francis Maude, the Cabinet Office minister, whom he said had been a "superb boss".
"It has been a great privilege to support Francis Maude in his relentless drive to make government more efficient," said Hurd. "Significant savings have been made, but I leave knowing that much radical reform is still needed and needs to be supported."
Hurd said he was looking forward to having time for other priorities, including the arrival of a new baby in the autumn and the opportunity to have greater freedom to lobby for more protection for his residents from the proposals for the high-speed rail link HS2, which will pass through his Ruislip, Northwood and Pinner constituency.