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 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.
They quickly became engaged in some of the most ferocious fighting UK forces had experienced since the Second World War.
MPs today 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.
"I absolutely accept that what we found when we had our forces on the ground was starkly different from what we had anticipated and been hoping for," he said.
"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."
He added: "We had always anticipated Taliban potential intent. What we probably underestimated was their capacity."
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.
General Sir Nicholas Houghton, Vice Chief of the Defence Staff, said there were a number of factors behind the serious upsurge in violence after British troops were deployed to Helmand in 2006.
These included the Taliban's portrayal of moves to eradicate opium plants as evidence that the UK forces wanted to destroy local farmers' livelihoods, the appointment of a new provincial governor which destabilised the tribal balance, and previous intensive American military operations which "whipped up" the situation.
The MPs also asked whether the killing of Osama bin Laden in his Pakistan hideout by US special forces had affected the situation in Afghanistan.
Gen Richards said there were indications that some parts of the insurgency were worried that the al Qaida leader's death could have an impact on their ability to raise money.
But he stressed that Washington had made no decision on whether to pull its troops out of Afghanistan any earlier as a result of bin Laden's killing.
"It is a net positive, but we don't really know yet how it will come through," he said.
Gen Richards said large numbers of Afghan soldiers and police were being recruited and trained, in line with the strategy for withdrawing foreign combat forces in 2014.
But he added that the jury was "still out" on whether their quality could be maintained after the pull-out of Nato troops.Reuse content