Nato must commit more troops and aid to Afghanistan or its mission may fail, MPs have warned.
The Commons defence committee said Afghanistan needed more support to develop into a stable democracy.
It raised serious concerns other Nato countries are not fully contributing to the conflict and warned failure in Afghanistan could undermine the very point of the transatlantic organisation.
The committee found the International Security Assistance Force (Isaf) is two battalions short of what Nato commanders say they need.
Isaf has nearly 37,000 troops deployed in Afghanistan. MPs said a far larger force, as well as greater assistance from development aid, would be needed to stabilise the country.
The report said: "We remain deeply concerned that the reluctance of some Nato members to provide troops for the Isaf mission is undermining Nato's credibility and also Isaf operations."
James Arbuthnot, the committee's chairman added: "The challenges facing UK forces in Afghanistan remain huge. The security situation in the south of the country is fragile to say the least and the cultivation of poppy is worse than ever.
"It is clear that an international presence will be needed there beyond 2009. If that commitment is to succeed, its size and strength must be very great, and in our view considerably greater than the international community is at present willing to acknowledge, let alone to make."
The mission is also being undermined by civilian casualties caused by Isaf troops. The committee warned this harms support for Nato and the Afghan government and fuels the insurgency.
The Conservatives have been warning for some time that a lack of commitment from Nato partners threatens to undermine the mission. They seized on the defence committee report as a "severe indictment" of the government's handling of Afghanistan.
Tory defence spokesman Liam Fox said: "Our military successes, achieved at considerable loss of life, are being undermined by the failure to sustain popular Afghan support as a result of slow progress in bringing on reconstruction programmes on a larger scale."
Defence secretary Des Browne welcomed the report, which he said recognised the UK was performing well against a challenging background.
He added: "I agree with the Committee's assessment that Nato nations should do more to meet the shortfalls in requirements. The UK continues to lobby other nations to provide more in terms of military and non-military resources."
The Liberal Democrats argued Gordon Brown must pull British troops out of Iraq in order to meet demands in Afghanistan.
Lib Dem defence spokesman Nick Harvey said: "This is an operation that Nato can ill afford to lose and yet coordination between international actors remains poor.
"While the operation is getting tougher and the number of casualties is climbing, our forces are still lacking the basic equipment they need to do the job."