MI5 and Scotland Yard, aided by GCHQ, have identified Jihadi John, the British man believed to have carried out the beheadings of two American journalists and British aid worker David Haines

David Cameron has signalled he is ready to dispatch Special Forces into Syria if it will bring Islamist executioner ‘Jihadi John’ to justice.

The Prime Minister made clear he would not necessarily wait for MPs’ approval before giving the order – if there was a chance of catching the masked killer with the British accent who appears in IS beheading videos.

Mr Cameron said: ‘In order to catch Jihadi John, you have to know where he is, right? We know he is in Syria.

'I very clearly reserve the position that if you need to act immediately, either to secure a vital British interest or to prevent a humanitarian catastrophe, you would act first and go to the House of Commons afterwards.’

British intelligence and security agencies are working closely with the FBI and CIA to find Jihadi John and his group of kidnappers in what is now a high-profile murder and counter-terrorism investigation.

MI5 and Scotland Yard, aided by GCHQ communication monitoring, have identified the British man believed to have carried out the beheadings of two American journalists and British aid worker David Haines.

This information has been shared with a team of more than a dozen US investigators from an FBI Joint Terrorism Task Force, from the US agency’s counter-terrorism command, who are reported to have flown to the UK in the past few weeks.

As The Mail on Sunday revealed last week, the jihadi nicknamed ‘John’ by the hostages he was holding – because he and the other British jihadis were known as The Beatles – is understood to come from a suburb of South London. It is believed his family members have been interviewed by Met officers and an FBI agent.

Meanwhile, teams from MI6 and CIA have deployed field officers in Northern Syria to help pinpoint the exact location of the group of kidnappers. These teams have picked up valuable intelligence gathered by their agents operating within IS ranks.

The US Senate has offered a £6million reward for any information that will lead to the capture of ‘John’.

But IS supporters are attempting to impose a social media blackout in a bid to frustrate US and British investigators and bombing missions.

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A message sent by the terrorist group yesterday warned its fighters and supporters from using Twitter, Facebook and other social media over fears they were giving away its forces’ positions.

IS fighters are engaged in a number of key battles in Syria and Iraq where they are being targeted by Coalition strike aircraft.

The message, distributed on Twitter by IS’s media arm using the hashtag Blackout Campaign, said the blackout was to be imposed immediately and that anyone who breached it will be ‘exposed and disgraced’.

David Cameron has said he will send in SAS troops to Syria before asking the permission of Parliament if such a move would secure a vital British interest or prevent a humanitarian catastrophe

Another IS member explained: ‘Do you know that a simple piece of information you type here [can] lead to abort the mission or the Mujahideen killed.’

This week a group of anti IS activists from the IS stronghold of Raqqa in Syria reported a new crackdown on anyone using cameras or social media.

The group, called Raqqa Is Being Slaughtered Silently, claim that IS police had taken control of the city and ‘look for anyone who takes pictures or videos of the city so they can arrest and execute him’.

Throughout the bombing campaign the reign of terror inside the city has continued with crucifixions, beheadings and punishments carried out in the main square.