Campaigners have demanded that the Ministry of Defence clarify whether it permits the use of lethal force by drones against suspected terrorists anywhere in the world.

Human rights groups have long suspected that, under certain exceptional circumstances, the UK will target threats outside conflict zones – a legally grey area.

The new defence secretary, Gavin Williamson, has also hinted that such a policy, similar to the one operated by the US, exists. In a recent interview, Williamson said: “My view is a dead terrorist can’t cause any harm to Britain.” He added: “We have got to make sure that as [Isis] splinters and as terrorists disperse across Iraq and Syria and other areas, we continue to hunt them down.”

The MoD’s joint doctrine on unmanned aerial systems (UAS), published in September, stated: “Arguments against using unmanned and remotely piloted aircraft are centred on worries that systems will be misused, or used illegally.

“They may also arise from the recent UK, and other states, practice of targeting suspected terrorists outside of the armed conflict itself and the meaning and application of a state’s right to self-defence.”

The government has been reluctant to confirm what the policy entails as this would open it to legal challenges, and, when recently updating its drone doctrine, omitted any reference to targeting outside conflict zones.

“The government appears to be in complete chaos over this,” said Jen Gibson, the “drones and kill-list projects” lead at Reprieve, a nonprofit organisation of international lawyers and investigators.

“Unless the government publishes its policy, we risk being dragged on President Trump’s coat-tails into unending conflicts around the world, from Yemen to Niger. The public has a right to know the government’s policy on taking lethal strikes in our name. And our armed forces deserve the protection of greater legal clarity and guidance.”

In 2015 the then prime minister, David Cameron, acknowledged that the use of an RAF drone to target a suspected Isis terrorist, Reyaad Khan, in Syria, represented a “new departure” for the UK, which was not engaged in the conflict at the time.

The revision to the doctrine came after SNP MP Stewart McDonald asked the MoD for clarification.

Metadata shows that the new version of the doctrine was created when the MoD received McDonald’s email.

Defence minister Mark Lancaster wrote to McDonald blaming erroneous drafting and denied such a policy existed. “If the government does resort to the use of military force (which is not confined to the use of UAS), then the government will always act in accordance with its national and international legal obligations,” Lancaster explained.

McDonald remains unconvinced.

“A document of this nature will have gone through an incredible amount of vetting and redrafting, and will presumably have been cleared by senior officials, legal experts and ministers to ensure accuracy and accordance with the law, before it is put into the public domain,” McDonald said. “To claim that this was just an erroneous drafting error just won’t cut it.”

He added: “The government’s number one priority must be to keep us safe, and there are strict rules of engagement for the use of drones where national security is at risk. However the government must assure parliament and the public that it is acting in accordance with the law and attempts to brush this off are not good enough.”

Chris Cole, of Drone Wars, which campaigns against armed drones, said the MoD needed to explain its position on drones as their use in conflict zones has become more prevalent. “Re-editing a brand new policy document to edit out an inconvenient truth is just the latest example of the MoD burying its head in the sand with regard to the legal and ethical concerns raised,” Cole said.