David Cameron is facing a legal challenge over the decision to target Islamic State (IS) terrorists in Syria - despite Parliament refusing approval for air strikes.
It was revealed earlier this month that two Britons were killed in an RAF drone attack near IS’s self-proclaimed capital of Raqqa.
The prime minister described the action as an “act of self defence”, the Press Association reported.
The Green Party's Caroline Lucas MP and Baroness Jenny Jones have joined forces with human rights charity Reprieve to take the first step towards a judicial review.
Lucas slated the actions as a secret “kill policy”.
She said: “The Government appears to have adopted a ‘Kill Policy’ in secret - without Parliamentary debate or the prospect of proper independent scrutiny. Sanctioning lethal drone attacks on British citizens is a significant departure from previous policy, as well as potentially unlawful, and it’s deeply concerning that it has occurred without appropriate oversight.
“By refusing to publish the legal basis for these attacks, the Government has created a legal and accountability vacuum. We need to be able to determine whether the attacks – and what they signify in terms of Government policy - meet the robust conditions set out in international and domestic law.
“I am part of bringing this case because if we want to be effective at countering terrorism then we must ensure we act lawfully. There are serious questions to be answered about the legality of the strikes, as well as the lack of robust oversight.
“Given the evidence from the USA, where former heads of defence and others have called their secret use of drones a ‘failed strategy’, it’s crucial that the UK’s actions to date and moving forward are subject to proper debate and scrutiny, particularly as its apparent new ‘Kill Policy’ goes beyond even what the US has been doing.”
In a pre-action letter to the Defence Secretary and Attorney General, lawyers for the politicians argue that the Government has either failed to formulate a "targeted killing policy" or failed to publish it. Both positions are illegal under domestic and international law, the letter says.
The letter reads: “"The Raqqa strike, and the intention of the Government to pre-authorise targeted killings in the future in countries where the UK is not at war, is of concern to the claimants and many others.
"The concern is heightened by the lack of clarity about the circumstances in which the Government reserves the right to kill British citizens outside of an armed conflict."
The letter says the Government has variously stated the drone strike was justified due to "potential", "direct", "likely" or "imminent" threats to the UK.
"Such a lack of clarity as to the test which is being applied by the Government in deciding whether to pre-authorise the targeted killing of British nationals or individuals overseas raises real and serious concerns over the lawfulness of the Government's past and expected resort to the use of lethal force," it insists.
"It is unclear what, if any, policies, procedures and/or safeguards are in place to ensure that this 'new departure' is only exercised in accordance with domestic and international law."
Reprieve has also blasted the strikes, with its legal director saying: “The Government has said it has the power to kill anyone, anywhere in the world, without oversight or safeguards. This is a huge step, and at the very least the Prime Minister should come clean about his new kill policy.
“Instead, we are seeing the UK follow the US down the dangerous path of secret, unaccountable drone strikes – a policy which has led to the deaths of hundreds of civilians in Pakistan and Yemen, without making us any safer.
The Government's Failure to Clearly Spell Out the Kill Policy Is Unlawful and Threatens the Bedrock of British Democracy
“Parliament and the public deserve to know what is being done in their name.
“It is disappointing that MPs are having to turn to the courts to extract even the most basic information on a policy which the Prime Minister himself has described as a ‘new departure’ for the country.”
Reyaad Khan, from Cardiff, and Ruhul Amin, from Aberdeen, were killed in a drone strike on 21 August.
Cameron said that Khan, who was the target of the strike, had been plotting “barbaric” attacks on UK soil.