The Tories want to replace government-financed volunteering programmes with a voucher system that would allow individuals to decide which charities benefit from state funds.
Cheryl Gillan, shadow secretary of state for Wales, last week told the Centre for Policy Studies that people would be able to use the vouchers - worth a certain amount of taxpayers' money - to generate funds for their charity of choice by volunteering for it.
"One of the trickiest challenges for us is to find ways of increasing state funding of charities without increasing the power of the state and diminishing the independence of the organisations we are funding," Gillan said.
"A possible way of doing this is to ensure that some of the money the state spends on the voluntary sector is allocated by citizens."
Volunteering England said in a statement: "Linking funding to volunteer numbers reflects a deep misunderstanding of volunteering and the voluntary sector. Some organisations need to involve a large number of volunteers to carry out their work, whereas others need very few. There is no reason for the latter to be punished by reduced funding."
CSV warned that the scheme could lead to a popularity contest in the sector.
"The likely result of a voucher scheme would be that donkey sanctuaries and cancer research would benefit hugely," said Dame Elisabeth Hoodless, executive director of CSV. "However, much-needed volunteer support for mental health service users, young offenders, abused children and frail older people might be neglected."