The open-ended promise came after days of headlines about complaints from British generals, first disclosed in The Times on Saturday, over shortages of essential equipment.

Treasury sources told The Times that there was no cap on military spending, in a clear sign that ministers now believe the mission to extend the Afghan Government’s writ in the south will be tougher and more expensive than thought.

Mr Blair’s pledge also came after pressure from MPs to explain why British troops did not receive helicopter support in a battle with the Taleban last week because the aircraft were too busy elsewhere.

The Government signed effectively a blank cheque for the mission in Afghanistan as a leading development official told The Times that it would take “decades” to eradicate opium production.

Security was a pre-condition to creating markets for different crops to tempt farmers away from poppy-growing, the official said, and “large areas” of Afghanistan were far from secure. Mr Blair and Treasury officials intervened yesterday after military figures suggested that spending in Afghanistan had been limited to £1 billion over three years by Des Browne, now the Defence Secretary but formerly Chief Secretary to the Treasury.

Earlier the Prime Minister had told MPs on the Commons Liaison Committee: “We are constantly assessing what more we need in terms of personnel, equipment and resources.”

He added: “Anything they need to ask for in order to protect our troops, I will make sure they get. Our obligation to them is to get them what they need to do the job.”

Mr Blair said that he had not received requests for extra resources from the military. Defence sources said that they were under the impression that a strict limit applied.

The Treasury insisted that there had been a misunderstanding. It had allocated £800 million for operations in Afghanistan since 2001 and would continue to allocate funds as necessary. A source told The Times: “There very definitely is not a cap on spending.”

The Chancellor has emphasised in recent months his readiness to spend on security and to back operations in Iraq and Afghanistan.The reassurances from the two top men in the Government came after complaints that British troops in Helmand province were desperately in need of more air cover.Military sources viewed the pledge to give commanders in Afghanistan extra troops or equipment as welcome but unrealistic. One senior military source said that the deployment of the 3,300-strong battle group to Helmand had been costed by the Treasury, and £1 billion allocated from the reserves to pay for its deployment for three years.

The sources added: “It was Des Browne, in his capacity as Chief Treasury Secretary, who was in charge of the financing of the Afghan mission.”

The sources insisted that Britain’s contribution to the operation was driven, not by military needs, but by Treasury requirements. Other senior military sources said that the offer from the Prime Minister was also unrealistic for practical reasons. “We have two major operations running, in Iraq and Afghanistan. There would be serious problems if troop levels in Afghanistan had to be increased significantly,” one said.

Five labourers working for US forces in Afghanistan were murdered and six people wounded in a bomb blast in Kabul yesterday.

• The two special forces soldiers killed in an operation last week to seize four Taleban commanders in southern Afghanistan were Captain David Patton of the Parachute Regiment and Sergeant Paul Bartlett of the Royal Marines, the MoD said last night.

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