British military personnel could be prosecuted for killing civilians in drone strikes and risk becoming complicit in alleged war crimes committed by the US, an inquiry has found.

A two-year probe by the All Party Parliamentary Group on Drones revealed that the number of operations facilitated by the UK in Iraq, Syria, Yemen, Pakistan and Somalia has been growing without any public scrutiny.

As well as launching its own strikes, the Ministry of Defence is assisting operations by the US and other allies that could violate both national and international law, it said.

Professor Michael Clarke, chair of the parliamentary drones group, said the UK was working with countries including the United Arab Emirates, Saudi Arabia and Qatar that “do not work on standard Nato rules”.

“As the trend of British personnel being embedded with foreign forces increases there is a danger they will find themselves complicit in drone strikes that are not legal on our terms,” he told The Independent.

“If we’re prepared to be a bit lax, which we are, it will get considerably worse as the use of drones proliferates.”

Professor Clarke, the former director-general of the Royal United Services Institute (Rusi), said any airstrike must be proportionate, with care taken to avoid civilian casualties.

“If civilians are going to get killed for a strike that ‘might be helpful’, that’s not good enough,” he added.

'White Widow' Sally Jones 'killed by US drone strike in Syria'

“Anecdotal evidence shows that operations that would previously not regarded as worth it are now, because drones are cheaper and lower the risk to the attacker.”

The UK currently has a range of small unmanned surveillance aircraft flying alongside a fleet of 10 MQ-9 Reapers, which are set to be replaced in 2024 with more than 20 Protector drones.

Military leaders told MPs that drones have become “a normal part of the business of gathering intelligence and conducting precision strike operations” and their use will continue increasing.

The killing of British Isis militants Reyaad Khan and Ruhul Amin in 2015 marked a departure from Britain’s previous practice, seeing the British jihadis droned in Syria without parliamentary approval.

They were followed by the British militant known as Jihadi John, who was killed in an American drone strike supported by UK intelligence, and several other jihadis whose deaths have been revealed by relatives or in domestic terror cases.

Professor Clarke said the government put forward “weak and inconsistent” legal arguments to justify the operations.

“’Arguably lawful’ is just not good enough,” he added. “No one objected because everyone was very glad to see the back of Jihadi John, but behind that the principles being compromised are very important.”

The government says the targeted militants constituted a significant threat to the UK but has not presented any information that would allow MPs or parliamentary committees to make a judgement.

A now deleted line in the MoD’s Joint Doctrine Publication on Unmanned Aircraft Systems stated that the UK had “a practice of targeting suspected terrorists outside of the armed conflict itself” and the defence secretary, Gavin Williamson, has stated his wish to “hunt down” suspected terrorists in “Iraq and Syria and other areas”.

The drones group found growing evidence that Britain is taking on military commitments to assist allies without parliament’s authorisation.

Syria bombings: US, UK and French military air strikes wreckage Show all 13 1 /13 Syria bombings: US, UK and French military air strikes wreckage Syria bombings: US, UK and French military air strikes wreckage The wreckage of the Scientific Studies and Research Centre in the Barzeh district, north of Damascus, which was targeted by the US, UK and France air strikes. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage of the Syrian Scientific Research Center surrounded by papers and rubble. AP Syria bombings: US, UK and French military air strikes wreckage Firefighrers extinguish smoke that rises from the damage. The Pentagon says none of the missiles filed by the U.S. and its allies was deflected by Syrian air defenses, rebutting claims by the Russian and Syrian governments. AP Syria bombings: US, UK and French military air strikes wreckage The wreckage of part of the Scientific Studies and Research Centre compound . AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Part of a building collapsing, surrounded by the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damaged to the Scientific Studies and Research Centre compound. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Lt. Gen. Kenneth McKenzie, the director of the Joint Staff at the Pentagon, also says there also is no indication that Russian air defense systems were employed early Saturday in Syria. AP Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier sprays water on the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Syrian state news agency SANA reported several missiles hit a research centre in Barzeh, north of Damascus, "destroying a building that included scientific labs and a training centre". AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage. AP Syria bombings: US, UK and French military air strikes wreckage Damage to the Scientific Research Center building that was hit by the strikes. EPA Syria bombings: US, UK and French military air strikes wreckage The Scientific Studies and Research Centre was one of the targeted buildings by the US, UK and France. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damage to the centre. EPA

“The current government does not consider parliamentary approval necessary when providing assistance to allies,” it concluded.

“As cooperation is likely to increase in the future, this approach leaves an expanding oversight and accountability gap. Moreover, it leaves UK personnel and ministers vulnerable to criminal liability in allies’ unlawful strikes.”

Because the use of force outside conflicts Britain is directly involved with is not protected by combatant immunity, British servicemen and women can be prosecuted for murder.

“There is growing concern that the UK is likely supporting a drone programme where the US commits unlawful acts,” the report said. “This Inquiry has found that the support provided by the UK constitutes the provision of material assistance to a state that appears to be violating international law.”

American forces operate four bases in the UK, while Britain operates and flies military drones from partner bases around the world.

Britain both shares personnel and intelligence with allies that can be used in the commission of strikes, and its personnel pilot American drones.

The report said the situation was “deeply problematic” because of the lack of oversight, while almost every request for information is “categorically dismissed”.

The government has refused to detail its policy on targeting or the process of launching drone strikes to parliament – or what is defined as a “combatant”.

While American forces have confirmed the death of more than 900 civilians in airstrikes on Isis strongholds in Iraq and Syria since 2014, the UK puts its own total at one after more than 1,700 strikes in those countries, and claimed a single operation caused civilian casualties in Afghanistan.

Mohammed Emwazi, known as Jihadi John, was killed in a US drone strike facilitated by the UK

Professor Clarke called the British government’s claims “ridiculous”, adding: “They don’t look for evidence and they don’t try to look for evidence.”

The parliamentary group called for proper mechanisms to identify civilian casualties, which are frequently reported in Isis territory and other areas where no British forces are present on the ground.

It warned that drone strikes can be used as propaganda by terrorist groups, while local residents experience psychological trauma and economic loss that continues to drive the cycle of violence.

MPs were also concerned about a shift in legal interpretation on a country’s right to defend itself against “imminent” attack, which the US has applied broadly to suspected terrorists worldwide.

In January last year, the attorney general suggested Britain had adopted America’s “dangerously expansive” interpretation of the right to self-defence.

Green Party co-leader Caroline Lucas, who sits on the drones group, said the government has “adopted a ‘kill policy’ in secret, and without parliamentary debate or the prospect of proper independent scrutiny”.

Labour MP Clive Lewis said: “Parliament is mandated to hold military policy and practice to account, but that role is slowly being hollowed out. When it comes to scrutinising Britain’s drone warfare, we’re kept in the dark.”

Adam Holloway, a Conservative MP and co-chair of the group said: “Taking back control means nothing if parliament has no mechanism to scrutinise the big questions of war,” he added. “There is an opportunity to set an example to the world through our drones policies: the government should seize it.”