Crack unit: The SAS troops have undergone intense training for the mission and have now flew to a secret base in the Middle East

The SAS has launched a dramatic mission to kill Jihadi John – the Islamic State terrorist responsible for beheading British hostages Alan Henning and David Haines – in the biggest Special Forces operation since the September 11 attacks.

Official Government sources confirmed that SAS soldiers flew to a secret base in the Middle East last week.

Travelling in groups of three and four, the crack troops boarded commercial flights dressed in civilian clothes.

They mingled with unsuspecting businessmen and tourists to maintain operational security.

After weeks of intense preparation at the regiment’s barracks in Hereford, the elite soldiers are now waiting for the order to strike Islamic State targets.

It is understood that part of the SAS mission could also include an attempt to rescue the remaining hostages, including British photographer John Cantlie.

The Mail on Sunday has been made aware of more detailed information about the mission but has agreed with Government officials not to publish it for security reasons.

However, we can reveal that the soldiers are sharing a remote desert location with British intelligence agents and communications experts who are understood to be tracking Jihadi John’s movements and also intercepting IS radio and telephone messages.

Final approval for any SAS mission inside Syria – where it is believed the hostages are being held – would have to be granted by the Prime Minister. David Cameron told Parliament in September that he would seize any opportunity to launch a rescue attempt.

Last night, an SAS source described to this newspaper how, to respect political sensitivities in the region, British troops were keeping a low profile.

He said: ‘The guys have left Hereford to carry out a strike raid on IS. They flew out there on commercial flights so that sorties by UK military aircraft can be kept to a minimum.

‘This is at the request of pro- Western leaders in the Middle East who are concerned about appearing to support the UK in the war against IS.

‘All the kit is ready and waiting for the guys and they’re on very short notice to strike IS.

‘It is more personal for the guys because Jihadi John is British and he’s killing British hostages. But now he’s basically a dead man walking.’

At the desert base, which is within striking distance of IS targets in Syria, the SAS troops are now carrying out final rehearsals for a raid and are using the most sophisticated weapons available to the British Army.

In addition to this arsenal, the SAS troopers are backed up by teams from 264 (SAS) Signal Squadron who are equipped with mobile satellites.

Using this equipment, the latest information about the whereabouts of Jihadi John and the remaining hostages in IS captivity is relayed to and from the frontline and back to SAS operational headquarters in Britain.

The SAS troops are also supported by a 17-man crew aboard a British spy plane called ‘Rivet Joint’. The crew’s job is to record conversations between jihadis as the aircraft circles thousands of feet above the battlefield.

These intercepts are fed to translators working with the SAS and are sent back to GCHQ, the Government’s secret communications headquarters. The messages between terrorists are matched against records of British jihadis believed to be fighting in Syria.

The SAS is also rehearsing raids using video footage provided by Unmanned Aerial Vehicles (UAVs), or drones.

These aircraft, which can fly for up to 24 hours and can climb to an altitude of 18,000ft, fly over IS strongholds and provide up-to-the-second pictures of the enemy’s hideouts. This live information will play a vital role in any British Special Forces operation to strike Jihadi John and rescue the hostages.

While he seems to speak with a distinctive London accent, it remains unclear just how much is known about Jihadi John. In September FBI director James Coney announced that the agency had established his real identity. However, no more information has been released.

Since his reign of terror began in August, Jihadi John has murdered four hostages – Americans James Foley, 40, and Steven Sotloff, 31, and Britons David Haines, 44, and Alan Henning, 47.