The two British ISIS jihadists killed in Syria when they were blitzed by a drone controlled by RAF pilots 3,000 miles away gave away their location when they phoned friends in the UK.

Reyaad Khan, 21, and Rahul Amin, 26, were killed when one of them made a phone call allowing security staff at GCHQ in Cheltenham to intercept the conversation and pinpoint their location in the Syrian desert.

British intelligence agents on the ground then confirmed which car the pair were travelling in and relayed the information back to controllers 3,000 miles away at RAF Waddington in Lincolnshire.

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Reyaad Khan, 21, (pictured lefT) and Rahul Amin, 26, (right) were killed when one of them made a phone call allowing security staff at GCHQ in Cheltenham to intercept the conversation and pinpoint their location.

The Sun on Sunday reports defence bosses then gave the go-ahead to bomb the men with Hellfire missiles from an unmanned MQ-9 Reaper drone.

The pair were being watched by the SAS, MI6 and the CIA for more than a month before the August 21 attack and an SAS team were drafted in to record the men's movements while GCHQ experts were ordered to monitor their phone conversations.

RAF experts operated the controversial £10million Reaper unmanned aerial vehicle (UAV) from a hi-tech control hub at RAF Waddington.

The Sun reports Defence Secretary Michael Fallon was briefed and gave the attack the go-ahead just minutes before the drone hit its target as the unsuspecting jihadists travelled in a vehicle in Raqqa, capital of the so-called Islamic State.

The paper revealed they were wiped out by a laser-guided anti-tank Hellfire missile after one of them made a phone call on an encrypted mobile phone.

David Cameron stunned MPs last week when he revealed the UK had used military force in Syria without parliamentary authority against the Britons.

Reyaad Khan, 21, from Cardiff, (left) and Ruhul Amin, from Aberdeen, (right) appeared together in an ISIS recruitment video last year aimed at luring jihadists to Syria and Iraq

‘There was a terrorist directing murder on our streets and no other means to stop him,’ said the Prime Minister.

A third man, Junaid Hussain, was killed three days later by a US drone in a joint operation with the UK.

The revelation that Khan, 21, from Cardiff, had been assassinated in the first RAF drone strike against a Briton triggered claims of extra-judicial killing.

But Mr Cameron insisted the attacks were an act of self-defence and not a softening-up exercise to persuade Parliament to vote for further military action against Islamic State targets in Syria.

Mr Cameron said Khan and Hussain were orchestrating attacks on ‘high-profile public commemorations’ over the summer from the IS stronghold of Raqqah.

Targets included VJ Day last month and the VE day commemorations of May 8.

The commemorations were attended by scores of war veterans and dignitaries including the Queen.

Other events in the fanatics’ sights are understood to have been Armed Forces Day in June, which marked the murder of Lee Rigby by two Islamist extremists, and the 100th anniversary of the Anzac landings at Gallipoli in April.

Ex-Conservative Attorney General Dominic Grieve said the strikes were ‘draconian’, adding: ‘I strongly suspect it will lead to a legal challenge.’

Kate Hudson of the Campaign for Nuclear Disarmament said it was an extra-judicial killing.

Reyaad Khan, 21, of Cardiff, in a picture he posted online with the caption 'Doing a lil training...#IS'

Asked if he would do it again, the Prime Minister told MPs: ‘If it is necessary to safeguard the UK and act in self-defence, and there are no other ways of doing it, then yes, I would.

'The choice we were left with was to think, this is all too difficult, throw up our arms and walk away, and then wait for the chaos and terrorism to hit Britain, or to take action in the national interest.’

The Prime Minister insisted that the drone strike was 'entirely lawful' after taking legal advice from the Attorney General.

'We were exercising the UK's inherent right to self-defence. There was clear evidence of the individuals in question planning and directing armed attacks against the UK.

'These were part of a series of actual and foiled attempts to attack the UK and our allies.

'And in the prevailing circumstances in Syria, the airstrike was the only feasible means of effectively disrupting the attacks planned and directed by this individual.

'So it was necessary and proportionate for the individual self-defence of the UK.'

Khan regularly boasted of his exploits with ISIS on social media and regularly posted pictures online

However, political opponents raised concerns about the decision to launch military action in Syria without first seeking permission from the House of Commons.

The Sunday Times reports today military bosses insisted lawyers were present during every stage of planning the covert attack to ensure it was legally 'watertight'.

The paper reports the operation needed a legal strategy to make sure international laws were not broken during the controversial attack.

Intelligence sources told the paper there was a 'significant amount' of concern over the legality of the operation to kill British citizens in a country that Britain is not at war with, but an SAS legal advisor said reassured them the attack was 'legally sound'.

Khan, who was previously a straight A student at a school in Cardiff, became a poster boy for ISIS when he appeared in a propaganda video entitled 'There Is No Life Without Jihad' in June last year, urging Westerners to join the war.

Like many ISIS fighters, he was a prolific Twitter user, frequently posting graphic images and bloodthirsty posts.

In one, he posted gruesome images of the corpses of alleged ISIS victims, tweeting: ‘Guys we caught and executed. This is how they looked less than an hr l8er [sic]’.

He also laughed about witnessing the ‘longest decapitation ever’ and boasted in another: ‘Executed many prisoners yesterday.’

But his Facebook page, which also lists him as a Chelsea FC fan who enjoyed playing computer games FIFA 12 and Call of Duty, reveals how rapidly he transformed from a normal teenager to a bloodthirsty jihadi fighter.

After appearing in the propaganda video with a Kalashnikov assault rifle against his shoulder, his mother said she believed he had been ‘brainwashed’ into joining ISIS.

Today the sister of another jihadist begged the Prime Minister to spare her brother from a similar drone attack because she also believes he has been brainwashed.

Abu Rumayash al-Britani has previously called for blood to be spilt on British streets and said the executions carried out by Jihadi John were 'justified'.

But speaking to the Sunday Times his younger sister, Konika Dhar, said families should be able to plead with jihadists before the government launches drone attacks against them.

She said: 'I want the prime minster to spare my brother because he is there as a result of brainwashing.