Most of the world's biggest aid donors are failing to make public what their spending achieves, according to a study released on Wednesday that ranks China, the United Arab Emirates and Japan as the worst performers.
Publish What You Fund, a campaign group, assessed the transparency of 45 organisations worldwide that spend at least $1 billion a year in aid to poor and crisis-affected countries.
Britain's foreign ministry said it could not release details around some projects in higher-risk countries and conflict zones for security reasons
One in four either failed to describe the projects they funded at all or described them in technical language that the public would struggle to understand, while only one in three reported the results of such projects.
"This makes it very difficult for watchdogs, partner country governments and donors themselves to understand what is working and if the promised results are being achieved," said Catherine Turner, the head of Publish What You Fund.
China's Ministry of Commerce, which manages the country's foreign aid spending, was the worst performer, followed by the foreign ministries of the UAE and Japan.
The Chinese embassy in London said China does not attach political strings to foreign assistance or interfere in the internal affairs of recipient countries.
A UAE foreign ministry spokesman said that it is engaging with a different set of transparency rules. Japan's foreign ministry did not immediately respond to requests for comment.
Without transparency, we risk undermining public trust in our life-saving aid
Overall, the report said most organisations had made improvements in recent years, with 95 per cent now publishing information to an internationally agreed standard. However, it said some data released was of poor quality or incomplete.
The Asian Development Bank (ADB) and the United Nations (UN) Development Programme took first and second positions as the most transparent donors, with Britain's Department for International Development (DFID) ranking third best.
"Transparency is a critical aspect of effective development work and we are glad to be at the forefront of it," Takehiko Nakao, ADB president said in a statement.
The United Nations Office for the Coordination of Humanitarian Affairs (OCHA) and the foreign ministries of New Zealand and Britain were also among the lowest scorers.
"Without transparency, we risk undermining public trust in our life-saving aid," said Romilly Greenhill, UK director of The ONE Campaign, an advocacy group.
OCHA said it published its financial data daily and that it was a mistake to measure its performance against a scoring system designed for long-term development aid, as the agency dealt with emergencies.
A New Zealand foreign ministry spokesman said it is committed to transparency but recognised the need for improvement and was taking steps to do so.
Britain's foreign ministry said it had made changes to improve transparency and was ensuring taxpayers' money was spent well but could not release details around some projects in higher-risk countries and conflict zones for security reasons.
"Foreign Office aid is tackling poverty and conflict and boosting prosperity in some of the world's poorest countries," a spokeswoman said.
Earlier this month, British lawmakers raised concerns that aid money from a fund managed by the foreign ministry was being spent to develop China's film industry and museums, rather than on helping the world poorest.