Charities are losing more than £30m a year by paying too much for foreign currencies, according to a new report from development finance charity Stamp Out Poverty.
Charities that buy less sought-after currencies, such as Kenyan shillings, at spot prices from their banks are often heavily overcharged, according to the Missing Millions report.
David Hillman, coordinator of Stamp Out Poverty, said his organisation wanted to see aid organisations invite competitive tenders for all currency transactions they performed.
"There is a hidden loss here," he said. "We believe that aid agencies are not pressing for the best prices because they don't realise there's a problem.
"People on the ground in a country don't know they could have got a better price, and donors haven't realised their money could go further."
He said the Department for International Development, which funds many British third sector organisations that work overseas, should take steps to improve the situation.
He said he would like to see DfID and large overseas charities working together to produce comprehensive best practice guidance on the subject.