Britain's war effort in Afghanistan is being hindered by a number of frontline troops too fat to fight, according to a leaked Army memo.

The Ministry of Defence confirmed today it had directed military chiefs to ensure units are following Army fitness policy after concerns were raised over a "worrying trend of obesity".

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The Army needs to "reinvigorate a warrior ethos and a culture of being fit", according to the leaked memo apparently sent to all Army units and obtained by the Observer newspaper.

The memo from Major Brian Dupree, of the Army physical training corps in Wiltshire, said basic fitness policy "is not being carried out".

Units were routinely failing to fulfil the Army's basic fitness regime of two hours of physical exercise a week, he added.

The memo leaked to the paper said: "The numbers of personnel unable to deploy and concerns about obesity throughout the Army are clearly linked to current attitudes towards physical training."

There are 3,860 Army personnel classified as PUD - personnel unable to deploy - with a further 8,190 regarded as being of "limited deployability" for medical reasons, it was reported.

Major Dupree added: "The current Army fitness policy states that to be fit to fight requires a minimum of two to three hours of physical activity per week. It is clear that even this most basic policy is not being implemented.

"To cope with the demands of hybrid operations in Afghanistan and future conflicts the Army needs personnel with that battle-winning edge that sustains them through adversity. It is clear this message has been diluted recently and this attitude must change.

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"The increasing PUD list and concerns over obesity in the services are clearly linked to this indifferent attitude."

An MoD spokesman said: "Following a review of recent evidence, direction has been given to the chain of command to take action to ensure units are following the Army's fitness policy."

The revelations came on the day a hard-hitting report on British efforts in the war-torn country by a committee of MPs was published.

Since deploying to Afghanistan in 2001, the UK has suffered from "significant mission creep" with an ever-growing list of responsibilities, including drugs, human rights and state-building, which have made it more difficult for the Government to explain the purpose of Britain's mission, said the report by the cross-party House of Commons Foreign Affairs Committee.