The umbrella organisation believes the commission will come under intense pressure during the consultation on its draft guidance from fee-charging public schools, lawyers and media pundits who are opposed to some of the criteria in the document.
The draft guidance says "people on low incomes must be able to benefit" from charities, including those that charge fees. This has been disputed by charity law firm Farrer & Co, which argued that it was inconsistent with case law.
Pete Moorey, parliamentary and media manager at the NCVO, said the umbrella body supported the commission's position on people on low incomes and was in total agreement with the strong line taken by the commission on fee-charging organisations.
"Fees and access to services must be at the heart of public benefit," said Moorey. "Without that, the case for charity will be undermined.
"Our concern is that you never get a lot of noise from organisations saying this is a good thing. It's always those that are likely to press the commission to water it down that will get the exposure."
In the draft guidance, the commission proposes that charitable private schools would have to do more to keep their charitable status than simply allow local clubs to occasionally use their facilities. It invites feedback on this and other criteria through a consultation that ends next week.