Paul Moore, strategic director for the north of England at Action for Children, said he was worried that a culture of lowest price rather than best value would take hold in local and central government, and that charity programmes could be cut back as a result.
"One of our main concerns is that when you get cutbacks in government spending, it tends first to affect preventive programmes," he said.
"This costs money in the long run, and we need to ensure we can show the Government that that's what happens.
"The people normally affected when programmes are cut are the hardest-to-reach, who cost the country the most money in the long run. We need to show that it makes no sense to do that.
"At Action for Children, we have worked hard on sophisticated models that show our cost benefit, and we're able to quantify the difference that we make."
Anne Blackmore, head of policy at the NCVO, said the Government's plan to spend its way out of recession should help charities. "This is a time the sector can really show its mettle," she said. "We can use this opportunity to demonstrate that the sector provides better value than anyone else."
Ralph Michell, policy advocate at chief executives body Acevo, said the Government would have to spend to cope with higher demand for services, and that the efficiency of the charity sector meant more money should be directed towards it.