Pulling British troops out prematurely from Afghanistan could "dangerously weaken" remaining forces, MPs said.
The Commons Defence Committee said David Cameron's plan to withdraw by the end of 2014 could undermine the international coalition's strategy.
And the MPs said they were still not yet convinced the troops now in Afghanistan had sufficient helicopters.
Defence Secretary Dr Liam Fox said while there was "still much to do", the pull-out target was achievable.
On a visit earlier this month to Afghanistan, the prime minister announced the withdrawal of 500 troops, cutting total force numbers to 9,000 by September 2012.
He said he wants British soldiers to end combat operations in the country by 2014.
On Sunday, Bamiyan province became the first area in the country to be handed over to Afghan security forces from Nato.
The formal process of transfer of power to Afghan control is due to be completed by the end of 2014.
But in their report, the MPs say there are still concerns about how ready the Afghan national army and police are to take over security and that withdrawal of British troops must depend on the situation on the ground.
They also said that the government's scope for any more troop reductions was "necessarily limited" in the short term, as a more significant withdrawal would have to involve a complete battle group - a "dangerous move".
"It is important that the government's clear determination to withdraw combat forces should not undermine the military strategy by causing the Afghan population to fear that the international coalition might abandon them or by allowing the Taliban and others to think that all they have to do is bide their time until International Security Assistance Force (Isaf) withdraws," the report said.
'Fighting for their lives'
The Ministry of Defence (MoD) is criticised for failing to warn ministers of the dangers facing troops when they were first deployed to Helmand province in 2006 under Tony Blair's government - a time when British soldiers were still fighting in Iraq.
The MPs say that for three years British forces lacked the necessary numbers and equipment after senior commanders in the UK told the then defence secretary John (now Lord) Reid that those on the ground had what they needed.
The MPs said it appeared "unlikely" commanders had sought ministerial authorisation for a change of tactics which saw British troops "fighting for their lives".
This tactical change should have gone to cabinet for endorsement, the MPs said.
"We are disturbed by the fact that the secretary of state was being told that commanders on the ground were content with the support they were being given in Helmand when clearly they were not.
"We regard it as unacceptable that hard pressed forces in such a difficult operation as Helmand should have been denied the necessary support to carry out the mission from the outset, and that this shortage had not been brought to the attention of ministers."
Committee chairman James Arbuthnot MP told the BBC: "Communications between the military and the politicians were poor, the warnings were inadequate at best and weren't passed properly up the line.
"Some of the decisions that were made in 2006 - for example to go out to the forward operating bases in Sangin, and Musa Kala and Now Zad - were things which really should have been put to the cabinet for decision before they happened.
"But they weren't, they were put to the ministers after they'd happened in a way which should never be allowed to happen again."
The MPs also said they are still not convinced that troops now in Afghanistan have enough helicopters after previous assurances from the government were later proved wrong.
Responding to the report, Dr Fox said it was clear that mistakes had been made in the lead-up to and during the deployment to Helmand in 2006.
He said troop levels had increased since 2009, as had the number of helicopters available to Isaf forces.
He added that Isaf was now ahead of targets to build and train the Afghan army and police.
"While there is much still to do, we are on track to achieve our target of ending UK combat operations in Afghanistan by 2015," he said.
"We will not abandon Afghanistan and as the prime minister has made clear the UK will work to further develop the ability of Afghans to look after their own affairs by leading the Afghan National Army Officer Academy amongst other things.
"My highest priority is ensuring that our service personnel are given all the support and equipment they need to do the job asked of them."
Robert Fox, from the Centre for Defence Studies at Kings College London, told the BBC that the greatest focus should be on the political mistakes rather than the actions of soldiers.
"The responsibility in the end is political. What worries me about this report and it's not generated by it at all - it's doing its job - is that now there's a feeding frenzy: let's blame the soldiers, let's push off, it's all over, it's their fault, they got it wrong.
"This is too simplistic and it's very dangerous."