Defence Secretary Liam Fox has issued a blunt warning that Britain and its coalition allies in Afghanistan need to "hold our nerve" and show the resolve to "see the job through".
Speaking in Washington, Dr Fox said that the "natural impatience" to bring the troops home as soon as possible should not be allowed to cloud the important national security interests they were defending.
Premature withdrawal of Nato forces would, he said, be a "shot in the arm" to violent Islamic extremists around the world and a "betrayal" of the sacrifices made by British forces.
Afghanistan could be plunged into civil war and its nuclear-armed neighbour Pakistan dangerously destabilised, while the determination of the West to tackle international jihadists would be called into question.
His comments, in a speech to the right-wing Heritage Foundation, come just days after David Cameron promised to bring home British forces by the time of the next general election in 2015.
The Prime Minister's remarks alarmed some Conservative MPs who warned that he was in danger of sending mixed messages to the Taliban about Britain's willingness to stay the course in Afghanistan.
Dr Fox acknowledged that there could not be an "indefinite" international military presence in Afghanistan.
"We want the Afghans to assume increasing responsibility for security in the next five years. We need, therefore, to get the job done," he said.
However he stressed that there was still important work to be done by the international coalition before that could happen.
Although al Qaida had been driven into the border areas of Pakistan, it continued to pose a "real and significant threat" to the West.
