Legal action charity Reprieve accused Britain today of bankrolling executions in Iran by funding the country's crackdown on illegal drugs.
Britain has given £3.6 million to Iranian anti-drug programmes since 1998 in an attempt to stem the flow of narcotics into Europe.
Iran has executed an estimated 12,000 people for drug offences since the 1979 Islamist revolution - and Reprieve said links between aid and executions were "not hard to establish."
Reprieve investigator Maya Foa said: "It's outrageous that Britain, which is supposed to be committed to the abolition of capital punishment, should in fact be funding executions for drug offences in Iran.
"Hundreds are being hanged every year, including children, vulnerable people and innocent scapegoats. That Britain should have played a part in this tragedy is shameful."
Most international aid to Iran is focused on equipment and training for its anti-narcotics police.
But the success of law enforcement agencies is measured by the number of arrests, which will "very likely" lead to executions, Reprieve said.
More than 1,200 people were executed for drug offences in Iran between 2007 and 2011.
Drug crimes accounted for 82 per cent of executions in 2011 - up from just 28 per cent in 2007.
Iran executes more people per capita than any other country.
A Foreign Office spokesman said Britain has funded no United Nations Office of Drugs and Crime programmes in Iran since 2007.
"The British government takes human rights very seriously and strongly opposes the use of the death penalty in all circumstances, including for drugs offences.
"We regularly condemn Iran on its abhorrent use of the death penalty."