Beattie, the former Scotland and British Lions rugby international, claimed ­national and local government was failing to take the problem seriously.

He said: “Because it’s not a public order issue, because they’re not puking up through alcohol or giving themselves lung cancer through smoking, it’s hidden from view and, therefore, not a sexy topic as far as our leaders are concerned.

“These girls are sitting ­quietly at home in chairs watching television, getting fatter and killing themselves. It’s unbelievable.”

Beattie, who chairs the review panel into the ­recommendations of the Physical Activity Task Force set up by the previous Scottish Executive, was speaking as the panel prepares to publish

its report.

He decided to speak out in advance of publication after figures on a Scottish Government website revealed that physical activity among girls after they started secondary school dropped from 63% in 2003 to 56% in 2008.

The task force set a target that would have increased activity by 1% a year, so the actual differential between the target and the present ­figure is 12%.

Beattie said: “If the trend continues over the next few years, that could drop another 7%. Then what do we do?”

He said all the political parties had backed the targets set by the task force but, five years later, young women were being let down.

“We’re alone in Europe in having this crash in young girls’ physical activity and it includes everything – walking, any social exercise. This demands action – to use smoking as an example, it’s much more important to be a physically active smoker than it is to be a non-smoking, non-physically-active person in terms of health.”

Beattie, whose son John is expected to play for Scotland against Argentina at Murrayfield today, said he was the wrong person to be championing the cause because he was a middle-aged man.

“Women should be up in arms about this because we are letting them get sick.”

He said schools had a part to play through better PE courses, but the key was ­promoting local initiatives.

“Research shows that sports celebrities, pop stars and political figures have little impact on young women, with 80% of the influence on a young girl and how she lives her life coming from her peers and the people in her neighbourhood. So you need lots of local initiatives to get influential people in local areas, especially deprived areas, to encourage healthy activities.”

Asked if there was any other objective he had in mind, Beattie said: “I’d like to see Alex Salmond get fit.”

Nanette Mutrie, professor of sport psychology at Strathclyde University, said there had been little progress in improving activity among teenagers for 20 years.

She said: “For young girls there appears to be a decline. This implies a timebomb of poor health as these girls grow up with greater risk of obesity, type-two diabetes, osteoporosis and depression than the more active girls.”