Schools will be expected to offer pupils two hours of PE a week under new government guidelines published yesterday, but no time limit has been set on achieving the goal.

Maureen Watt, Minister for Schools, said tackling childhood obesity was now a "high priority" for the Scottish Government.

Pupils could also get the chance to take part in physical activities in break times, lunchtimes and at the end of the school day, she said.

They will also explore issues such as the pressures placed on them by food advertisers and the political debate which surrounds food and health today.

Children will be encouraged to take their healthy knowledge home too, as they will also be taught how to read food labels so that they can make better choices about the food they eat - and help their parents shop more healthily.

However, Labour's schools spokesman Ken Macintosh said questions still needed to be answered about how the two-hour PE commitment was to be met.

The 2007 SNP manifesto had stated: "To help Scottish children develop the habit of physical fitness, we will ensure that every pupil has two hours of quality PE each week, delivered by specialist PE teachers."

But yesterday's guidelines said: "The Scottish Government expects schools to continue to work towards the provision of two hours of good quality PE for every child every week."

Mr Macintosh said: "The SNP have realised that offering regular PE to children goes a long way to improve their physical and mental health, setting them up for an active lifestyle in adulthood.

"However, as with class sizes and school buildings, important questions remain unanswered about how the government is going to meet this commitment.

"I sincerely hope that it does not become another of the SNP's broken promises."

Ross Finnie, health spokesman for the Liberal Democrats, described the measures as "hardly radical".

"Despite accepting that childhood obesity poses a major threat to the future health of our nation, the SNP's target falls far short of what is needed to target this ticking timebomb," he said.

The guidelines, published by the Scottish Government and Learning and Teaching Scotland, which develops curriculum materials, are part of the ongoing reform of what is taught in schools - the so-called Curriculum for Excellence.

In addition, Ms Watt revealed that measures which aim to take sweets and fatty foods off the menu in schools will come into effect from August in primaries.

Under the new regulations - which are part of legislation passed by the previous Scottish Executive - schools will have to offer at least two portions of fruit and vegetables every lunchtime.

Pupils will also get free drinking water throughout the day and deep-fried food will be limited to three items a week.

The regulations, which come into effect from August next year in secondary schools, will also see sweets and fizzy drinks removed from schools, while chips must only be served as part of a nutritionally balanced meal.

Children will also be encouraged to learn about all aspects of healthy eating, and will be taught how to understand the information on food labels, so they can make healthier choices about the food they eat outside of school.

Ms Watt outlined why the government was taking action, and said: "Obesity is an increasing problem and poses a serious risk to health.

"That's why tackling the problem, particularly in early life, is a high priority and why we're determined to help children develop a taste for healthy eating at the earliest possible age.

"As part of our national food policy we will be encouraging people to develop healthy eating habits that will benefit them for the rest of their lives and help tackle the serious problem of obesity.

"Giving children guidance on what's good for them and how to make good food choices is essential in taking this forward."

Speaking about the expectation that schools will offer youngsters two hours of PE a week, Ms Watt said: "This will give children and young people an important foundation for participation in experiences in physical activities and sport in preparation for a healthy and fulfilling lifestyle."