For the past decade British intelligence has systematically co-operated with US and Yemeni agencies in the covert war against al-Qaeda in Yemen, a VICE News investigation can reveal.

Long characterized as a unilateral policy of the United States, VICE News has found that through the use of double agents, surveillance, and electronic tagging, Britain's Secret Intelligence Service (SIS, commonly known as MI6) assisted in identifying and locating human targets for American drone strikes from 2010 onwards.

The covert war in Yemen has killed up to 1,651 people, including up to 261 civilians, according to the Bureau of Investigative Journalism. Many more have been injured.

"We had a targeting list with names that we could pursue," explained Stephen Seche, former US ambassador to Yemen. "It was very useful for both [Britain and America] to sit and help triangulate what we were hearing from our different sources."

'Britain provided "unique" capabilities via its network of human intelligence'

A former senior CIA official, responsible for operations in Yemen, noted that Britain provided "unique" capabilities via its network of human intelligence. "If you look at what capabilities each side has, that starts to tell you something about precisely where the actionable intelligence is coming from."

It was worrying that the UK's role had not been made public, said British MP David Davis, chair of the All-Party Parliamentary Group on drones. "The involvement of the British state is something that the government ought to make plain to parliament," he said.

"If we know we're handing intelligence over which will be used in a killing then we ought to be confident that it meets our own rules and guidelines."

A SIS team also mentored Yemen's National Security Bureau (NSB) in a Joint Operations Room to observe and fix al-Qaeda targets ahead of American drone strikes. Ali al-Ahmadi, NSB director between 2012-2015, told VICE News: "SIS co-operated with us a lot in mentoring our surveillance team, which prepares for raids and arrests, observation of targets, and fixing them. That was really one of the reasons for the success of the NSB."

'Our station people were pretty shit-hot'

Al-Ahmadi confirmed that the British mentoring was both "theoretical and operational" and that "the surveillance teams were a British specialism."

A former British official who served in Yemen summed it up bluntly: "Our station people were pretty shit-hot."

British trainers maintained a low profile, however. Colonel Yahya Saleh, who between 2001 and 2012 commanded Yemen's Central Security Forces, who were also trained by the UK, noted: "They stipulated that we couldn't take their photos, or mention their names; even when we were honoring the American trainers the British avoided having their names mentioned."

After the US began to use a tactic known as "signature striking" — firing on unidentified groups of military aged males engaged in "suspicious" behavior — in 2012 British military trainers were forbidden from sharing intelligence with American forces, to ensure compliance with Britain's counterterrorism rule of law.

'Once they are seconded the MoD loses any control over what they get up to'

However, according to American, Yemeni, and British officers who served in Yemen, members of Britain's Special Reconnaissance Regiment (a special forces unit) were seconded to SIS, mentoring the NSB and Yemen's Political Security Organization. This placed those soldiers under the control of the Foreign and Commonwealth Office (FCO), freeing them from the Ministry of Defence (MoD) regulations and allowing them to covertly continue to feed into the US drone program as intelligence agents.

A British official noted that "once they are seconded the MoD loses any control over what they get up to."

In response to a parliamentary question about UK involvement in the Yemen war by MP Tom Watson in July 2014, the FCO said: "Drone strikes against terrorist targets in Yemen are a matter for the Yemeni and US Governments. We expect all concerned to act in accordance with international law and take all feasible precautions to avoid civilian casualties when conducting operations."

The number of strikes derived from British intelligence is a closely guarded secret, but a senior CIA officer responsible for operations in Yemen confirmed that intelligence sharing on targets was routine.

Up to nine strikes are believed to have derived from one British agent, including a strike on a collection of farms known as Wadi Rafad on May 6, 2012, which killed Fahd al-Quso, a senior field commander in al-Qaeda in the Arabian Peninsula (AQAP) who had participated in the bombing of the USS Cole in 2000.

'A very fast way of signing up a lot of people to our enemies'

The strike also killed 19-year-old student Nasser Salem Lakdim, who was tending to his crops in the adjacent field. Nasser's father came rushing home to find his son in pieces. "It was horrifying, I can barely describe it," he told VICE News.

In another strike based on intelligence from the same British agent, on March 30, 2012, a US drone fired three missiles at a car in the center of a Yemeni town. In 2015, GCHQ internal documents said the strike targeted "a doctor who pioneered using surgically implanted explosives." Along with a fellow AQAP militant, the strike killed a 60-year-old civilian bystander and injured six children, on their way to play soccer.

Nabeel Khoury, US deputy chief of mission in Sanaa from 2004 to 2007, wrote in 2013 that: "Drone strikes take out a few bad guys to be sure, but they also kill a large number of innocent civilians. Given Yemen's tribal structure, the US generates roughly 40 to 60 new enemies for every AQAP operative killed by drones."

Davis concurred: "Killing people from a clear sky who are guilty of nothing is a very fast way of signing up a lot of people to our enemies."

Last November Harriet Harman MP, chair of the Joint Parliamentary Committee on Human Rights, told an audience at the Royal United Services Institute that, "We must look at governance where we hand intelligence to others, such as the US, leading to a strike. We are culpable in that process."

Responding to the VICE News allegations, an FCO spokesperson said: "We have previously provided counter-terrorism capacity building support to the Yemeni Security Services to increase their ability to disrupt, detain, and prosecute suspected terrorists in line with Yemeni rule of law and international human rights standards. Following the closure of the embassy in Sanaa in February 2015 we suspended this activity. We continue to work with regional and international partners to tackle the threat posed by terrorist organizations including AQAP and Daesh-Yemen [using an alternative acronym for the Islamic State group]and to build regional capacity on counter terrorism."

An MoD spokesperson said: "The MoD does not comment on special forces operations, or intelligence matters."

Follow Namir Shabibi on Twitter: @nshabibi