The Mod said a British Apache aircraft fired on a position which was thought to be held by enemy forces but instead injured members of a British ground patrol.

Three were seriously wounded and six more were classified as walking wounded after Wednesday's incident in Helmand province, the MoD said.

One of those injured has been flown back to the UK but is in a stable condition. Two others remain at Camp Bastion medical centre where they are being kept in for observation.

The six others have been discharged and have returned to their duties. Next of kin have been informed.

A statement from the MoD said: "On July 9 at 12.27pm, a routine British patrol requested fire support from a British Apache when they encountered enemy forces near Forward Operating Base Gibraltar.

"After successfully engaging one enemy position, the Apache fired upon another position which the crew believed to be held by enemy forces.

"However, in the confusion of a rapidly changing situation and in what is a challenging environment, it would appear that friendly forces were mistaken for the enemy and as a result three members of the patrol were seriously wounded and six more were classified as walking wounded.

"The seriously wounded personnel were moved by helicopter to Camp Bastion medical facilities.

"An investigation is under way into how the incident occurred and it would be inappropriate to comment until that investigation is complete."