British troops preparing to deploy to Afghanistan face "fundamental tension" over their mission, MPs have warned.
More than 3,300 troops will be based in Helmand province
The Commons Defence Committee said attempts to crack down on drug lords in the area, one of the troops' missions, could result in increased violence.
The MPs also expressed concern that the 3,300-strong force lacked close air support and transport helicopters.
The Army is preparing to deploy the full task force, led by 16 Air Assault Brigade, to start operations in June.
The troops are intended to restore security and stability in the southern region of Helmand, which has recently seen outbreaks of violence by the Taleban.
But they may also be asked to support anti-narcotics operations in the province, which has the highest concentration of opium poppy-growing in the country.
BBC world affairs correspondent Nick Childs said MPs are not yet sure what the troops will be required to do in Afghanistan.
"But, according to them, it's likely that the more successful the deployment is in impeding the drugs trade, the more it'll be attacked, and the worse security will get," he said.
The MPs' report said the task force "faces significant obstacles, and the security situation is fragile.
"The opium trade flourishes and the livelihoods of many people rely on it. There is therefore real tension between the tasks of achieving security and reducing the opium trade.
"It is likely that the more successful the deployment is at impeding the drugs trade, the more it will come under attack from those involved in it."
The committee said that the Taleban, who are believed to number as many as 1,000, were becoming more active in the drug trade.
MPs also questioned the Ministry of Defence's decision to withdraw a squadron of RAF Harrier GR7 fighter-bombers from an air base at Kandahar.
The commission said support from US and Dutch warplanes may not be sufficient.
The six Chinook and four Lynx helicopters currently assigned to the detachment may not be enough to support the troops in terrain which often impossible to cover, even with back-up from US and Dutch helicopters, according to the MPs.
They were also worried that the RAF's Hercules transport planes were not fitted with adequate "defensive aids suites" to prevent them being shot down from the ground.