Katie Ghose says it's natural for charities to operate as both service providers and campaigners, but warns that increasing demands on them to take on contracts present risks to their advocacy role.
"You can't divorce services from campaigning - and nor should you," says Ghose, chief executive of the Electoral Reform Society.
"It's no surprise that many charities start off delivering services and then want to speak out about their experiences. It's natural for them to look upstream for the cause of the problem."
She cites law centres and Citizens Advice as examples of charities that successfully combine the dual roles. But she says there is a danger that, as charities get drawn ever more tightly into providing services by contract, they have less time to advocate - and perhaps less inclination, if the people they are campaigning against are the same as the people hiring them.
The growth in 'contract culture' means charities could spend all their time providing services and filling in forms while growing wary of "biting the hand that feeds them", she says.
Ghose outlined her concerns at last month's National Council for Voluntary Organisations campaigning conference, where she said the sector had to make the case for its dual role as advocate and service provider. Politicians should be reminded, she said, that campaigning is not a nuisance, but a way to improve services.
She says that, besides the big society, the forthcoming referendum on the alternative vote is a key issue for campaigners this year.
Ghose, who was director of the British Institute of Human Rights before taking up her current role in October last year, is chair of the Yes to Fairer Votes campaign, which supports abolishing the first past the post system.
She says giving voters the chance to select more than one candidate would encourage politicians to speak to a wider range of people.
"It could mean that local campaigners will have more of an opportunity to put their issues on the map at election time," she says.
New Charity Commission guidance says a charity can campaign on the issue only if the outcome of a referendum is likely to directly affect the delivery of the charity's objects.
But Ghose says the Yes campaign is looking to voluntary organisations without charitable status for support. Greenpeace and Friends of the Earth are among those pledging support.
Ghose is a barrister with a background in human rights and immigration. She was director of the British Institute of Human Rights and worked for Age UK and Citizens Advice before becoming chief executive of the Electoral Reform Society last year.
The Electoral Reform Society wants to change the current first past the post voting system, which it claims produces unrepresentative governments. A referendum will be held in May on whether to introduce the AV system, in which voters choose three candidates in order of preference.