Image caption Reyaad Khan (L) and Ruhul Amin were killed in August

The use of drones to kill UK citizens abroad is being legally challenged by two Green Party parliamentarians.

Earlier this month, David Cameron announced an RAF-operated drone had killed two Britons linked to so-called Islamic State in Syria, describing the action as an "act of self defence".

MP Caroline Lucas and Baroness Jones have now sought permission for a judicial review of the policy, claiming that "targeted killing" is unlawful.

It is backed by campaigners Reprieve.

Britain is participating in air strikes against IS fighters in Iraq, but not Syria - after the Commons refused to approve the action two years ago.

But in what was the first targeted UK drone attack on a British citizen, Reyaad Khan, from Cardiff, and Ruhul Amin, from Aberdeen were killed by a remotely piloted aircraft on 21 August in Raqqa, Syria.

Khan, the target of the strike, had been plotting "barbaric" attacks on UK soil, the prime minister told MPs in a statement.

Media playback is unsupported on your device Media caption David Cameron explaining the action to MPs earlier this month

He insisted the action was lawful despite Parliament not having authorised airstrikes against Islamic State targets in Syria.

He said the attorney general had been consulted and agreed there was a "clear legal basis" for the strike.

But Ms Lucas and Baroness Jones - the party's sole representatives in Parliament - say there has been a lack of parliamentary scrutiny or approval of the policy and argue there have been conflicting and partial accounts of the justification in statements.

In his statement to MPs on 7 September 2015, David Cameron also said:

The strike had been approved at a meeting of "the most senior members" of the National Security Council, and authorised by Defence Secretary Michael Fallon

The UK acted under the "inherent right of self-defence" contained in the Charter of the United Nations, based on evidence from intelligence agencies

British security forces had foiled six attempted terror attacks in the UK in the past year

The UK had a "moral responsibility" to help people fleeing the Syrian conflict and would accept up to 20,000 refugees from Syria over the next five years

In a letter to the Ministry of Defence and the attorney general's office, they argue that the government has either failed to formulate a policy or, if it has a policy, failed to publish it. Either eventuality, they argue, is unlawful.

"If any pre-authorised and targeted killing can be lawful, they must be carried out under a formulated and published Targeted Killing Policy which ensures transparency, clarity and accountability for such use of lethal force," the letter says.

'Kill policy'

The drone strike is expected to be the subject of an investigation by the Intelligence and Security Committee of the House of Commons, now chaired by former Attorney General Dominic Grieve.

Reprieve, an international human rights charity, said the government's justification of the strike effectively gave it "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," said its legal director Kat Craig.

She told BBC Radio 4's Today programme it was effectively a "wholesale adoption" of the US government's "so-called targeted killing programme".

The drone strikes have been backed by Conservative MPs, who said the individuals targeted posed a direct threat to national security.

But Labour has called for the attorney general's advice to be published, something ministers say there is no precedent for.