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.

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.

‘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. 

(dailymail.co.uk)