The failure of some Nato countries to provide troops in Afghanistan is seriously undermining the organisation's credibility as well as the operations of the International Security Assistance Force, according to the Commons Defence Committee.

In a report raising as many questions as it answered, the Defence Committee yesterday joined the chorus demanding that more Nato countries pull their weight.

The report also urges the Ministry of Defence to make greater efforts to increase the provision of appropriate helicopters to UK forces and sufficient trained air and ground crew, and the committee agreed that the UK helicopter operations in Afghanistan were not sustainable at the present intensity.

James Arbuthnot, committee chairman, said: "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 the 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 make.

"Our service personnel, not least our helicopters and ground crews, are doing a great job in extremely demanding conditions. But they are working to the limit and that cannot go on forever. The MoD must do more or else the whole mission could be undermined."

The Tories claimed that the report was a severe indict-ment of the government's handling of the situation in Afghanistan, and that it made a mockery of John Reid, the former Defence Secretary, who claimed only last year that British troops could be in Afghanistan for three years "without a shot being fired".

Dr Liam Fox, the shadow defence spokesman, 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."