American officials have consistently played down the number of civilian casualties, insisting that precision targeting meant the annual number was in the “single digits”.

In his report, Mr Emmerson cited Pakistani government statistics showing 2200 people had died in drone strikes during the past decade in Pakistan, of which at least 400 were civilians.

“Officials indicated that, owing to underreporting and obstacles to effective investigation, those figures were likely to be an underestimate,” he wrote.

Figures have only recently been made available in Afghanistan, where the UN says 31 civilians have been killed in drone strikes in the past 18 months. In Yemen the figure is 12-58, according to Mr Emmerson, while statistics are not available from Iraq or the Nato operation in Libya in 2011.

Drone attacks are a source of widespread anger in Pakistan, where the government publicly condemns the missiles strikes as an assault on its sovereignty.

Yet there is evidence that politicians and military commanders have privately given the green light to missions.

Supporters insist the Reaper and Predator aircraft are responsible for cutting a swath through the senior leadership of al-Qaeda and other terrorist groups in Pakistan.

Mr Emmerson, a British human rights lawyer, also criticised the US for failing to release its own casualty data and called for greater clarification of legal justification of the strikes.

Critics of the programme have long called for greater transparency.

Joshua Foust, a security and intelligence analyst, said the report marked an important contribution to the debate over drones.

“Emmerson identified all of the key ambiguities that make discussing drone strikes so difficult: the secrecy surrounding them, the international legal ambiguities, and the fact that they actually can, in theory, reduce civilian casualties,” he said.

“Hopefully it will catalyse more informed debate.”