It's a complex task involving more than the military, says General Sir Mike Jackson
The military mission in Afghanistan has failed to deliver on its promises - as troops have too many tasks, MPs say.
The House of Commons Foreign Affairs Select Committee says "mission creep" had brought too many responsibilities, including fighting the drugs trade.
Poor government planning and a lack of realistic strategy and clear direction undermined the mission, the MPs said.
The report said the focus should be on security, but Armed Forces Minister Bill Rammell said it always had been.
He also told Sky News the committee "wildly overstated" the amount of British resources put into the drugs fight and despite the army's co-ordination role, troops were "overwhelmingly" engaged in security.
The report said Britain's deployment to Helmand province was "undermined by unrealistic planning at senior levels, poor co-ordination between Whitehall departments and crucially a failure to provide the military with clear direction".
Mr Rammell said it was important to have a "comprehensive approach" to ensure better governance and delivery of basic services.
British forces experienced their bloodiest month in Afghanistan during July, with 22 troops killed during a major offensive against insurgents in Helmand.
Britain's objectives in Afghanistan should be realistic, tightly-defined and subject to regular formal assessment
The report warns the security situation, particularly in the south, could be expected to remain "precarious for some time to come".
The committee suggests the continued instability is due in part to the failures of the international community and criticised some of Nato's allies for delivering "much less" than had been promised.
"We recognise that although Afghanistan's current situation is not solely the legacy of the West's failures since 2001, avoidable mistakes - including knee-jerk responses, policy fragmentation and overlap - now make the task of stabilising the country considerably more difficult than might otherwise have been the case," it said.
The committee said government claims the Afghan drugs trade was a justification for the continued presence of British troops in Helmand were "debatable."
"We recommend that in the immediate future the government should refocus its efforts to concentrate its limited resources on one priority, namely security."
It also said Afghanistan was a "most critical and seminal moment" for the future of the Nato alliance as it was the first deployment outside its "area".
The report continued: "There is a real possibility that without a more equitable distribution of responsibility and risk, Nato's effort will be further inhibited and its reputation as a military alliance, capable of undertaking out-of-area operations, seriously damaged."
General Sir Mike Jackson, who was head of the Army when British troops went into Helmand province in 2006, said the report did not doubt the overall reasons for being in Afghanistan which were to "prevent terrorism and "achieve a degree of stability".
"Ourselves, the Americans, Canadians, Dutch, the Danes, the Estonians have all taken on a significant burden militarily."
Committee chairman Mike Gapes
He told the BBC's Andrew Marr show: "What the committee is saying is that the way in which the job is being done needs some improvement, it's a complex task and it involves far more than just the military."
He said the higher recent casualty rate had brought sharply into public focus the complexity of the challenge and that things were very different in 2006 when Britain was heavily involved in Iraq.
He also said he wants a review into compensation for injured troops to be independent following the Ministry of Defence's legal bid last week to try to cut compensation to two injured servicemen who developed subsequent health complications.
Committee chairman Labour MP Mike Gapes told BBC Radio 5 Live some countries were not "pulling their weight" in the international coalition, highlighting how some had sent "handfuls" of troops or none while others had sent thousands.
"Certain countries are taking serious casualties and are in the areas where there is very much more fighting and danger.
"Ourselves, the Americans, Canadians, Dutch, the Danes the Estonians have all taken on a significant burden militarily."
In the report Mr Gapes said the UK had taken on a "poisoned chalice" by assuming a lead role in counter-narcotics in partnership with the Afghan government.
He said the task had been "an impossible job for one country to do" and said it should be handed over to the International Security Assistance Force and the United Nations.
He added: "This issue of counter-narcotics, the heroin poppies, is a serious problem but it is not the main issue that we face in Afghanistan.
"The main issue is the security and the threat coming from, once again, becoming a terrorist base."
A Foreign Office spokesman said they would carefully consider the report's findings and respond to Parliament in the coming months.
"The Foreign Office looks forward to further discussions on Afghanistan and Pakistan with Parliament and all interested parties. These issues deserve the widest possible engagement."
Shadow foreign secretary William Hague said the report should be "a wake-up call to the government".
"It confirms what we have been saying for months: Britain's objectives in Afghanistan should be realistic, tightly-defined and subject to regular formal assessment," he said.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.