DAVID CAMERON is facing legal action after sanctioning a secret drone strike in Syria which killed two British citizens.

The Prime Minister was challenged yesterday to prove the strike was legal and come clean over whether it was part of a “kill policy.”

Green MP Caroline Lucas and peer Jenny Jones issued the ultimatum, telling the Tory he’ll face the courts if he fails to answer.

A pre-action letter issued to Defence Secretary Philip Hammond and Attorney General Jeremy Wright asks why the strikes announced two weeks ago were carried out without parliamentary approval or scrutiny.

And it argues the government has either failed to formulate a “targeted killing policy” or failed to publish it — both would be a breach British and international law.

Reyaad Khan and Ruhul Amin, both British citizens fighting with Islamic State, were killed in Raqqa by RAF drones on August 21.

The PM insisted the strike — branded an “extrajudicial killing” by campaigners — was an “act of self-defence” that could be repeated.

He said the Attorney General had been “consulted,” but has refused to publish the legal advice he received or allow any independent scrutiny of the legality of his decision.

Ms Lucas said yesterday that has left a “legal and accountability vacuum” over drone strikes.

“The government appears to have adopted a ‘kill policy’ in secret — without parliamentary debate or the prospect of proper independent scrutiny,” she said.

“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.

“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.”

The Green politicians are being supported by human rights charity Reprieve, which has represented civilian victims of US drone strikes.

Mr Cameron stressed that no civilians were injured or killed in the RAF attack.

But hundreds of civilians have been killed by US drone strikes despite CIA claims that none were harmed, according to Reprieve.

“As many as 1,147 people may have been killed during attempts to kill 41 men,” a 2014 report found.

Reprieve legal director Kat Craig warned: “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.

“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.”

A government spokesman would only say: “The Prime Minister has been clear that this action was legal, necessary and proportionate.”