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It is thought to be the best long range shot in the elite regiment’s history.

The sniper – a sergeant and veteran of the fighting in Iraq and Syria – killed the terrorist with one round from a huge .50 Calibre machine gun.

The bullet struck him in the chest with such force it ripped off his arm and shoulder, killing him instantly.

Sources say the gun will now be decommissioned and taken to the SAS headquarters near Hereford as a memento.

(Image: ALAMY)

The sniper was on a secret patrol in an Isis-controlled area of northern Afghanistan in June.

Militants have established a series of strongholds in the area. The SAS team in armed vehicles were watching an Isis base when they spotted the commander, said to be on a joint British and US “kill list”.

Although the patrol was equipped with sniper rifles, the troops believed the only weapon capable of hitting the target was the .50 Cal Browning mounted on one of the vehicles.

They asked for permission to engage the target and it was approved by a senior officer at the Joint Special Operations Command HQ in Kabul, the Afghan capital.

One source familiar with the operation said: “The .50 Cal has got a phenomenal range and is very accurate even though it is almost 40 years old.

“It can be fired on single shot. The sniper fitted a special sight to the machine gun and got a spotter to estimate the wind speed.

“He also took into account the heat of the day and the light. The image of his target was quite ‘watery’ because of the heat being given off from the ground.

“The Islamic State commander was briefing his men and clearly liked the sound of his own voice because he was standing still for a least 20 minutes while his fighters sat on the ground in front of him.

“The sniper knew he only had one chance. It took several seconds for the round to hit the commander who appeared to fly into several pieces. For a few seconds no-one moved. When they realised what had happened they got up and ran away.”

The Daily Star Sunday has learned that the killing of the IS commander is thought to have saved the lives of more than 20 people.

The .50 Cal was used as a longrange weapon during the Korean War in the 1950s but this is believed to be the first time the machine gun has been used in a purely sniping role by the British Army’s elite regiment.

A spokesman for the Ministry of Defence told us: “We do not comment on special forces operations.”