Britain's war effort is being hampered by the number of front-line troops who are too fat or unfit to be deployed to southern Afghanistan.

A leaked memo sent to all army units and obtained by the Observer reveals that basic fitness policy "is not being carried out" and highlights concern among military commanders over a "worrying trend of obesity" that is limiting the number of soldiers fit enough to fight in Helmand. Units are routinely failing to fulfil the army's basic fitness regime of two hours of physical exercise a week, it adds, and the army must "reinvigorate a warrior ethos".

To counter the problem, the army will introduce a "body composition measurement" policy this October to weed out overweight troops as well as enforcing a bare minimum of three physical training sessions a week.

"The numbers of personnel unable to deploy and concerns about obesity throughout the army are clearly linked to current attitudes towards physical training," states the emergency memo from Major Brian Dupree of the army physical training corps in Wiltshire. He warns that Britain's "operational effectiveness" is being undermined and that soldiers' lives could be placed at risk because some are unable to cope with the brutal conditions of Helmand province and the stresses of combat.

Last night a leading Conservative MP and retired colonel described the revelations as "disgraceful" at a time when commanders are demanding more British troops be sent to Afghanistan to hold ground recently seized from the Taliban during Operation Panther's Claw. Commanders have already sent an extra 125 troops to Helmand to replace those injured or killed in the offensive that saw the highest number of British casualties in Afghanistan since the conflict began.

Currently 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. The MoD cannot give a breakdown for how many of these are obese or simply unfit.

Dupree states in the memo dated 10 July: "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.

"The increasing PUD list and concerns over obesity in the services are clearly linked to this indifferent attitude." He concludes that the army has "not consistently maintained our standards of physical fitness" and needs to "reinvigorate a warrior ethos and a culture of being fit".

Patrick Mercer MP, head of strategy at the Army Training & Recruiting Agency, said: "This lack of personal fitness is a disgraceful state of affairs. The army is desperately undermanned anyway and for obesity to be a problem is extraordinary."

Dupree's report comes three years after the army relaxed its rules to allow recruits with a higher body mass index (BMI) to join after research found that two thirds of British teenagers were too fat to meet fitness requirements. Applicants with a BMI of 32 - two points above the World Health Organisation's definition of obesity - can now enlist.

An army board of inquiry in 2007 revealed how a soldier who died of heat stroke in Iraq was "at the higher level of obese". Investigators revealed that concerns were expressed about Private Jason Smith's BMI.

Dupree's report states: "The demands on time, which are acknowledged, are such that physical training has been regarded as something that can be cut from busy schedules. This approach cannot continue. Strong leadership is expected in this area."