Afghan police say they have confiscated more than 90kg of opium from drug smugglers posing as aid workers.
Afghanistan produces 75% of the world's illegal opium
The opium was found in two vehicles marked with fake insignia near the northern city of Mazar-e-Sharif last week, they said.
They are searching for three other vehicles they believe are being used for drugs.
A spokesman for relief workers said the find would further damage aid agency reputations in Afghanistan.
Aid workers have already complained that attempts by the US military to engage in community development by building clinics and sinking wells has compromised the neutrality of the aid organisations.
They are also worried by accusations by Afghan officials that they have squandered international aid.
Relief groups would now have to take extra security measures to avoid casualties, Nick Downie of the Afghanistan Non-Governmental Organisation Safety Office said.
"The best security strategies rely strongly on community acceptance," he said. "Unfortunately, the acceptance that has been built up over years of effort within local communities is eroding rapidly."
Afghanistan produces 75% of the world's illegal opium, according to the UN.
The UN warns that the industry is being used to fund terrorism and is threatening the country's stability.