The approach to the UK's mission in Afghanistan's Helmand province, in 2006, was flawed, according to a senior military officer involved in planning the deployment of troops.
Rear Admiral Chris Parry said Armed Forces leaders had "immature" ideas and had not expected to fight the Taliban.
But the Ministry of Defence (MoD) said the decision to deploy UK forces in Helmand followed "careful analysis".
Most of the UK's 294 Afghan fatalities have occurred in Helmand province.
Rear Admiral Parry was the MoD's director-general of development, concepts and doctrine in 2006, and played a major role in talks leading up to the initial deployment of around 3,300 troops to Helmand - a figure which has since increased to about 10,000.
He told BBC Radio 4's Today programme that those in charge of the mission had based their strategy on memories of missions in Borneo, Malaya and Northern Ireland and senior figures showed "considerable" resistance to "ditching the lessons from the past" rather than adopting the "radical and progressive ideas" which were needed.
He told Today: "At the time, I think we had an immature approach to what is now known as counter-insurgency.
"We didn't realise the complexity and the character of the context in which we were going to fight. In fact, we didn't envisage we were going to fight."
Rear Admiral Parry said the new government is "getting it right" in Afghanistan, adding only that "we need to go further".
An MoD spokesman said: "The decision to deploy UK forces to Helmand in 2006 followed careful analysis and comprehensive discussion within the MoD and across government.
"The new government has made clear that the mission in Afghanistan is imperative for our national security and its commitment to ensure that it succeeds."
Tory backbencher Adam Holloway, a member of the House of Commons Defence Committee, said the UK's strategy in the province had so far been "fatally flawed" and indicated that he expects Prime Minister David Cameron to shift his approach.
He told Today: "The guy's smart and he wants to get it right ...he has just taken over and you don't necessarily declare your hand as soon as you have started."
Mr Holloway also argued that it is time to seek deals with ousted Taliban leaders to pursue a loose form of local governance.
"The truth is that we have got to bring the people who lost in 2001 - as much of the hardcore Taliban as possible - into the political process and we have got to make a series of local deals with tribal leaders to get them to stop fighting and we have to bring the regional powers on board."
He said the al-Qaeda terror network's presence in Afghanistan had been smashed, and the task now was to separate mainstream Muslim communities from radicals.