Labour leadership favourite Jeremy Corbyn has attacked David Cameron for launching a drone strike which killed a British ISIS fanatic.

The veteran socialist said was 'unclear as to the point of killing' terrorist Reyaad Khan in the secret operation on August 21.

Mr Corbyn questioned whether the RAF airstrike which killed Khan and a fellow UK jihadist Ruhul Amin was legal and confirmed he would not have authorised the attack.

Jeremy Corbyn has attacked David Cameron for launching a drone strike which killed a British ISIS fanatic. He is the overwhelming favourite to succeed Ed Miliband as Labour leader after voting closed at midday today

The Prime Minister stunned MPs on Monday by revealing the UK had used military force in Syria without parliamentary authority and against a Briton.

Mr Cameron said: 'There was a terrorist directing murder on our streets and no other means to stop him.'

A third British fanatic, 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 entirely legal.

He 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.

Reyaad Khan, 21, from Cardiff, was killed in response to a 'direct threat' to the British people, the Prime Minister said. Ruhul Amin, from Aberdeen, was also killed in the attack on August 21

Another British fanatic, Junaid Hussain, from Birmingham was killed by a US drone strike late last month

PLOT TO 'KILL THE QUEEN FOILED' A WEEK BEFORE DRONE STRIKE An ISIS-inspired terror plot to kill the Queen was foiled just a week before the drone strike was launched by the RAF. Last month it emerged that intelligence services had uncovered a plan to use an Improvised Explosive Device to murder the Queen, dignitaries and veterans at a VJ Day parade. A major security operation was launched over fears of terrorists intent on causing carnage at the event on August 15 to mark the end of WWII in the Far East, during which 30,000 British troops were killed between 1941 to 1945. Downing Street refused to confirm which events were targeted by Khan. But David Cameron told MPs he was behind 'plots to attack high profile public commemorations, including those taking place this summer'. Advertisement

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.

However, speaking to ITV News today Mr Corbyn said 'the PM has some very difficult questions to answer about the legality of what he did'.

He added: 'I'm unclear as to the point of killing the individual by this drone attack.'

Mr Corbyn is the runaway favourite to be elected Labour leader on Saturday, despite warnings from senior party figures that he cannot win the next election.

The 66-year-old backbench MP today insisted that the Labour party would unite around him if he won the leadership.

He said he was 'very confident' that the party would come together to present a 'very strong opposition to what the government is doing'.

Under his leadership, he said, Labour would 'put across a serious radical alternative message'.

But Mr Corbyn's opposition to the drone strike on ISIS terrorists is likely to spark fresh concern among moderate Labour MPs.

Scores of Labour MPs are understood to be prepared to defy Mr Corbyn to back military action against ISIS, even if he orders the party to oppose it.

Mr Cameron this week confirmed he would stick to his pledge to hold a Commons vote – probably next month – before ordering the RAF to conduct bombing raids against Islamic State terrorists in Syria.

Lord McDonald, a former director of public prosecutions, said the killing of Khan was legal and justified. He said: 'It is appropriate to invoke the principle of self-defence and to target him.'

Khan, who was previously a straight A student 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.

David Cameron used a speech in the House of Commons to unveil details of an RAF drone strike which killed three jihadis in Syria, including two Britons

RAF Reaper drones had been carrying out surveillance over Syria but ministers had insisted it would require 'further permission' for them to use weapons in the country

Later attending St David's Catholic sixth-form college in the city, he was described by friends as a studious pupil who loved sports.

At the time of his disappearance, Khan's family said they were desperate for him to come home.

His mother Rukia, 41, pleaded: 'Please come back before it is too late. You are my only one son. I love you so much.'

But after arriving in Syria in late 2013 along with his friend, 21-year-old medical student Nasser Muthana, Khan changed beyond recognition.

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.'