The report, Funding for Sustainable Change, says 79 per cent of trusts have criteria that allow them to fund campaigning, advocacy and influencing activities, but only 30 per cent definitely fund them.
"A large number of funding opportunities exist," said Amy Rosser, project manager for the DSC. "But a lack of clarity from trusts is preventing voluntary sector organisations from accessing funds. Despite having the potential within their objectives to support these activities, most trusts don't state this clearly."
The report is based on a sample of 2,500 trusts in the DSC's Directory of Grant-Making Trusts 2007/08.
Helen Donohoe, head of campaigning effectiveness at the NCVO, said: "This is part of opening up the debate about the core role campaigning activity can have in delivering a charity's mission."
The grant-giving City Parochial Foundation is holding a conference for funders in November to address how trusts can improve their funding for campaigning.
Mubin Haq, director of policy at the foundation, said his organisation had made a more explicit commitment to funding campaigning over the past two years, and the number of charities approaching it for grants had grown as a result.
Ros McCarthy, a charity lawyer with Bates Wells & Braithwaite, said grant givers had been cautious about the legality of funding campaigning, but new Charity Commission guidance, CC9, made it clear that they were allowed to. "The next step is to draw up a model policy for grant givers so they can provide funds clearly and easily," she said.
Helen Shaw, co-director of Inquest, an organisation that campaigns on deaths in custody, said grant makers were getting better at explaining what they do.
"Some foundations have communicated on this very clearly, and a lot of others have been reluctant," she said. "There's a lot more talk about the best way to do it."
The DSC is working with the campaigning effectiveness team at the NCVO to encourage funders to support campaigning specifically.