Applicants with a body mass index (BMI) of 32 — two points above the World Health Organisation’s definition of obesity — will now be permitted to join up. This means that a 6ft recruit could weigh as much as 16 stone 11lb and still be allowed into the army.

Neither Martin Johnson, the former England rugby captain, nor Jonah Lomu, the All Blacks winger (6ft 6in and 6ft 5in respectively), would have qualified under the old rules, which specified a maximum BMI of 28. Both would now qualify.

An army spokesman said this weekend that the previous rules had almost certainly led to the rejection of recruits with the potential to become successful soldiers.

“We have moved the goalposts,” he said. “There is an argument that we were missing out on soldiers who, although on paper might seem to be obese, were in fact very fit and had a lot of upper body strength, like rugby players, for example.”

The attempt to broaden the range of recruits is also a tacit admission that both the recruitment and the retention of soldiers have suffered since the Iraq war.

Army chiefs admit that in addition to the unpopularity of the war, bad publicity associated with equipment shortages, allegations of abuse and courts martial have tarnished the army’s reputation and badly affected recruitment.

Army chiefs insist that no concessions are to be made on fitness requirements. All recruits are required to complete a series of physical infantry tests including the basic personal fitness assessment. To qualify, soldiers under 30 must be able to do 44 press-ups and 50 sit-ups in two minutes and then run a mile and a half in 10 minutes 30 seconds.

Brute strength is not always the most important characteristic. Many infantry soldiers are small and wiry and a man of Lomu’s build would find it hard to fit in a tank driver’s seat.

For other roles, strength is at a premium. The Royal Engineers require people with the power to build bridges by hand, while the artillery demands “good upper body strength . . . required to lift 44kg shells for prolonged periods”.

The new guidelines are part of an effort to counter a severe shortage of soldiers, which begins tomorrow with a three-month television and radio campaign costing £4m.

The change also reflects a growing body of research which suggests that BMI is only a crude indicator of whether a person is unfit or carrying excess fat. Technically, many leading athletes are massively overweight.

Expanding waistlines are no bar to rising up the ranks. General “Stormin’” Norman Schwarzkopf, the 6ft 3in American commander in the 1991 Gulf war, has a BMI of 30.2. making him technically obese and earning him the nickname of “the Bear”.

One reason for the rule change was that it was excluding many Fijians, who are much bigger on average than British recruits. Staff Sergeant Talaiasi Labalaba, a giant Fijian, became a well-known SAS hero after manhandling an artillery gun to fire shells into advancing communist rebels in Oman in 1972.

More than 1,800 Fijians currently serve in the army and many who want to join were being excluded under the old rules.

Publicly the army insists that it is not suffering shortages. But Brigadier Robbie Scott-Bowden, director of infantry, has cancelled planned redundancies, citing “manning deficiencies exacerbated by recent poor recruiting”.

There has also been an increase in the number of soldiers choosing to leave early, he said, adding: “It would appear to be counter-intuitive to be making soldiers redundant when the infantry is so undermanned.”

Additional reporting: Ben Schofield

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