Fair Exchange? Measuring the impact of not-for-profit partnerships, published this week by sustainability charity the Forum for the Future, said charities risked accusations of the same kind of 'greenwashing' that companies often face.
"Not enough is being done to communicate the measurable impacts of such partnership working," said the report, by Ben Tuxworth, director of strategy, and Florian Sommer, senior sustainability adviser at the charity.
It went on: "Even organisations claiming significant impact over many years do little to cite quantifiable evidence."
According to Sommer, NGOs should publish both quantitative and qualitative data to back up claims that they made a difference.
"If an NGO is working with a company to improve living conditions for young people, the young people should talk about the work," he said.
The report concludes that NGOs should develop a shared partnership code, which would set standards for reporting.
A spokesperson for Amnesty International, cited in the report, claimed it was sometimes difficult to quantify the role of NGOs in corporate dialogue.
"It can be difficult to assess how effective NGOs actually are when some of the most effective consultations are happening face-to-face behind the scenes," she said.
Environment charity WWF has partnerships with some 50 different companies, including HSBC.
Joss Tantram, business education manager at the charity, said: "We do need to take on board these standards. But we also need to get more comfortable with an understanding that the things we are doing are right, even if we can't count every bean."