The committee said "mission creep" had brought too many responsibilities
Armed Forces Minister Bill Rammell has defended the government's strategy in Afghanistan following criticism by MPs.
The House of Commons Foreign Affairs Select Committee said the military mission had failed to deliver on its promises as troops had too many tasks.
Mr Rammell told the BBC the UK had a "co-ordinated and comprehensive" strategy in Afghanistan.
But shadow defence secretary Liam Fox argued there was no co-ordinated strategy in Afghanistan.
The committee of MPs said that "mission creep" had brought too many responsibilities, including fighting the drugs trade.
It said the UK's deployment to Afghanistan's 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".
MPs also said UK troops should abandon their attempts to tackle the drugs trade and focus instead solely on security.
But Mr Rammell insisted there was already clarity of purpose in Afghanistan.
He told BBC Radio 4's The World This Weekend: "The scale of the challenge from the insurgents in Helmand province is greater than we'd anticipated, and we are responding to that.
"But nevertheless we are making progress, there is a co-ordinated, comprehensive strategy, it is about making Britons safer, it's about working as part of an international coalition.
"This isn't just the responsibility of this country or the UK government."
British forces experienced their bloodiest month in Afghanistan during July, with 22 troops killed during a major offensive against insurgents in Helmand.
The report warns the security situation, particularly in the south, could be expected to remain "precarious for some time to come".
The committee said government claims the Afghan drugs trade was a justification for the continued presence of British troops in Helmand were "debatable."
Its chairman Labour MP Mike Gapes said: "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."
He later 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.
Responding to the report, Mr Fox said: "The danger is that the military take a space and they hold it, but nothing comes in behind and therefore eventually the Taliban get another foothold."
Earlier shadow foreign secretary William Hague said: "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."
One member of the MPs' committee, former Liberal Democrat leader Sir Menzies Campbell, said the government had failed to set out the precise terms of the mission.
"We've been drawn into things like counter-narcotics strategy, nation building - and that, we believe, has dissipated our effort," he said.