Intelligence failings meant British soldiers were not prepared for the fierce Taliban resistance they encountered when they first deployed to Helmand, the head of the armed forces has said.
General Sir David Richards said UK forces "turned up a hornet's nest" when they moved into the violent province in southern Afghanistan in 2006.
"There was in some respects a failure of intelligence despite the efforts to get it right," he told the Commons defence committee.
Some 3,300 British troops serving with 16 Air Assault Brigade took over control of Helmand in May 2006 and quickly became engaged in some of the most ferocious fighting UK forces had experienced since the Second World War.
On Wednesday, MPs questioned top military leaders about why they failed to assess correctly the strength of the Taliban insurgency in the province.
General Sir Peter Wall, the head of the Army, agreed there "clearly was" a failure of intelligence, adding: "We were ready for an adverse reaction but we did not, to be fair, expect it to be as vehement as it turned out to be."
Gen Wall said every effort was made to establish a clear picture of the situation in Helmand before British troops were deployed there.
He said the British intelligence agencies were "actively engaged" and noted that there was even a former member of the mujahideen resistance who had fought in Afghanistan in his "gap year" on the staff of the military's Permanent Joint Headquarters (PJHQ).
Gen Richards said most of the information available related to the provincial capital, Lashkar Gah, which was "pretty positive and benign", rather than the enemy-held territory in northern Helmand.
"The crux of the problem was when we went into the north and arguably turned up a hornet's nest," he said.