Nine British soldiers were injured by “friendly fire” when an Army Air Corps Apache attack helicopter flying at close quarters targeted the troops by mistake during a clash with Taleban insurgents.

Three members of the British patrol were seriously injured in the incident, in Helmand province. They were taken by Chinook helicopter to the main British base at Camp Bastion, farther south in Helmand, where they received medical treatment.

Two remain in the camp hospital and are under observation. The third seriously wounded soldier is believed to have undergone an operation at Selly Oak hospital in Birmingham. His condition was described as stable last night. The six “walking wounded” have now been discharged and have returned to duties. All next of kin have been informed.

The soldiers, all of whom are believed to be members of 2nd Battalion, The Parachute Regiment, were on routine patrol south of the town of Sangin when the incident occurred, shortly after noon on Wednesday.

An initial “contact” took place with Taleban fighters close to Forward Operating Base Gibraltar, a British base near Sangin. The patrol called for close air support, which was provided by one of the eight Apache helicopters based at Camp Bastion.

“The Apache successfully engaged one enemy position,” Colonel Robin Matthews, the Helmand task force spokesman, told The Times. “The Apache then fired on another position, which the crew believed held enemy forces. In what was a confusing situation in a rapidly changing environment, friendly forces were mistaken for the enemy.”

FOB Gibraltar is a small British outpost established just before Christmas by Royal Marines as British forces sought to wrest control of a strategically important area known to soldiers as The Witch’s Hat east of the Helmand river.

The British Apache helicopter, which is armed with cannon and Hellfire missiles, had been providing close-air support for the soldiers as they engaged the Taleban. The Apaches have transformed the battlefield since they were first deployed to southern Afghanistan in 2006. They are a formidable weapon platform and often operate in support of ground troops, hovering a few yards away.

Separating friend from foe is one of their biggest challenges. This is the first time that a British Apache, flown by the Army Air Corps, has been involved in a “friendly-fire” incident.

In a speech in Washington yesterday, Des Browne, the Defence Secretary, said that there had been significant military progress in Afghanistan, which meant that the war could “no longer be won by the Taleban”.

He said that the stabilisation of Afghanistan, however, was being put at risk by liberal opposition to Islamic laws that “may not sit easily with our culture and norms — but do so with theirs”. He added: “A Western model would be inappropriate and unsustainable, but unless we address these challenges our efforts to create an effective police force will continue to be seriously undermined.”

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