The long-term future and reputation of Britain’s Armed Forces is at risk unless progress is made in Afghanistan, the two most senior officials at the Ministry of Defence warn in an internal document seen by The Times.

The pronouncement by Air Chief Marshal Sir Jock Stirrup, the Chief of the Defence Staff, and Sir Bill Jeffrey, the Permanent Secretary, leaves no room for the possibility of early withdrawal from Afghanistan. “Planning within Defence should be based on the assumption of a rolling three-year military commitment to Afghanistan, reviewed annually,” they say in a jointly signed document circulated as guidance to MoD staff preparing for next year’s defence review.

Their unequivocal statement of commitment appeared out of step with a more conditional speech on Afghanistan given by Gordon Brown yesterday. He was accused by the Opposition of sending out mixed messages and making empty threats after warning President Karzai, the Afghan leader, that he was not prepared to put the lives of soldiers “in harm’s way” for a government that did not stand up to corruption.

Mr Brown emphasised the importance of keeping the international alliance together in Afghanistan but then said: “We will succeed or fail together.” While insisting that British troops must stay, he said he had told President Karzai that he would forfeit the right to international support if he failed to root out corruption and improve his governance of the country.

Downing Street said Mr Brown’s words did not mean that British troops would be withdrawn if Mr Karzai failed. But they were a warning that the West’s patience with him was limited and that personal backing would be withdrawn if he did not meet the tests being set by Mr Brown and President Obama.

The internal MoD document, entitled Strategy for Defence, makes it clear that as far as the Armed Forces are concerned, the mission in Afghanistan has to succeed. “The International Security Assistance Force mission is of critical importance to the security of British citizens and the UK’s national interest, including the credibility of Nato, and to the reputation and long-term future of the Armed Forces,” the document says.

There are already concerns within the military that the premature British troop withdrawal from Basra in 2007 and the pullout from Iraq in July may have tarnished Britain’s reputation in the eyes of the American military.

Liam Fox, Shadow Defence Secretary, accused Mr Brown of giving out mixed messages and making empty threats: “We must put pressure on the Karzai Government to improve governance and tackle corruption, but if our mission in Afghanistan is a national security imperative, it can’t be conditional on the behaviour of others.”

In phone calls with Mr Brown since his re-election, Mr Karzai accepted the need for action on five key areas: security, governance, reconciliation, economic development and engagement with its neighbours. It is expected that the Afghan President will set out his plans to meet these goals in his inauguration speech on November 19.

Meanwhile, the soldier killed by an explosion in Sangin in central Helmand on Thursday was named as Serjeant Phillip Scott, 30, of the 3rd Battalion The Rifles.

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