London, Sep 24 (IANS): British Prime Minister David Cameron has faced a legal challenge over the decision to target Islamic State (IS) terrorists in Syria despite parliament refusing approval for airstrikes, the media reported on Thursday.

The prime minister revealed earlier this month that a Royal Air Force (RAF) drone had killed two Britons in an attack near the city of Raqqa, and described the action as an “act of self defence”, The Guardian reported.

Reyaad Khan, from Cardiff, the primary target of the RAF drone strike, and Ruhul Amin, from Aberdeen, were killed on August 21 in Raqqa.

Cameron told MPs that Khan had been plotting "barbaric" attacks against Britain and there was no other way of stopping him.

Green Party MP Caroline Lucas and peer Jenny Jones have joined forces with human rights charity Reprieve to take the first step towards a judicial review.

In a pre-action letter to the defence secretary and attorney general, lawyers for the politicians argued 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 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 letter said.

The letter said the government has variously stated the drone strike was justified due to “potential”, “direct”, “likely” or “imminent” threats to Britain.

Britain is carrying out airstrikes against IS in Iraq, but not in Syria after the House of Commons refused to approve the action.