UK aid money sent to Iran to fight drug smuggling is being used to pay for the executions of criminals, according to human rights campaigners.
Iran has historically been the leading recipient of UK anti-drugs assistance, receiving £3.6 million over four years through joint programmes with other countries.
Iran is second only to China in the number of executions carried out
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."
The majority of aid provided to the Islamic state by international governments is focused on improving the efficiency of its anti-narcotics police (ANP), which can include providing night-vision goggles, GPS and customs training.
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 in Iran between 2007 and 2011 for drug offences, while the proportion of total executions for drug crime has rocketed from 28% to 82% in that period.
The Reprieve report comes shortly after prime minister David Cameron defended the government's decision to ring-fence the Department of International Development budget and commit to spend 0.7% of GDP on aid programmes.
Cameron said that even while times were tough at home, Britain had a "moral obligation" to assist the poorest parts of the world.
But Foa said that "given the country's appalling record on human rights", there was real concern over how equipment and support provided by Britain to Iran is being used.
She said: "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."
Iran is a major transit route for drugs smuggled from Afghanistan through Pakistan to the Persian Gulf, Turkey, Russia and Europe.
The country, led by president Mahmoud Ahmadinejad, is frequently reported as having the largest seizures of opiates - such as morphine - in the world.
It claims to have invested more than a billion US dollars (£613m) in an elaborate series of earthworks, forts and deep trenches to channel potential drug smugglers to areas where they can be confronted by security forces.
Iran executes more people per head than any other country, with 12,000 estimated to have been executed in Iran for drug offences since 1979.
The UK has provided £3.6m in funding in to anti-drug programmes in Iran since 1998.
It jointly funded a project to promote intelligence-led investigations in Iran between 2010 and 2011, which led to the seizure of 23,633kg of opium, 1,490kg of heroin, 3,033kg of cannabis, 425kg of morphine and 110kg of crystal meth.
The £750,000 initiative coincided with the sharp increase in executions for drug offences, Reprieve said.
Another jointly funded project, between 2007 and 2010, resulted in £117,000 being spent on motorcycles, along with X-ray body scanners and mobile and satellite communications.
A Foreign Office spokesman said the UK has funded no United Nations Office of Drugs and Crime (UNODC) programmes in Iran since 2007.
He added: "We continue to raise with the UNODC, and other UN bodies, the need to ensure that counter-narcotics projects are compliant with international human rights and we have supported the publication of human rights guidelines for UNODC projects.
"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."