The letter from Ian Leggett, director of People & Planet, lists specific government programmes that prevent or restrict the sector's lobbying activities, and urges Miliband to educate Government colleagues about the benefits of charity campaigning.
The letter was prompted by an interview with Miliband in Third Sector last month, when the minister emphasised that the "voice and activism role" of the voluntary sector should not be undermined by the receipt of public money.
"If that's a role he wants to foster and support, he would do well to start by removing the funding obstacles that already restrict our activism," Leggett said.
He pointed out that there was "almost a contradiction" between Miliband's view and some government programmes. Examples in the letter include the Department for International Development's Development Awareness Fund. This restricts charities receiving funds from getting involved in initiatives that "involve direct lobbying of the UK government or of international organisations of which the UK is a member".
Leggett also referred to the Department for Environment, Food and Rural Affairs' Climate Challenge Fund. It stipulates grants "must not be used for direct political campaigning".
According to Leggett, campaigning is part of the diplomatic process and is often "the only way in which you can begin a dialogue" on issues such as climate change.