Large discrepancies in the way obesity is being tackled across Scotland are exposed today in a landmark report.

It found overweight patients face a postcode lottery accessing support to help them lose weight, while doctors do not even use the same measurements to decide which patients should be referred to weight management clinics.

Public health leaders are calling for a national strategy to fight the problem on the back of the research.

The study was commissioned by the Scottish Public Health Network (SPHN), which brings together directors of public health, amid concern obesity is the biggest health problem facing the western world.

Scotland has one of the highest rates of obesity, second only to the US on some league tables.

Widespread progress has been made improving school meals and encouraging people to take physical activity, according to the SPHN research. However, there are greater gaps in the drive to stop adults becoming fat and the treatment available for obese patients.

A survey of GPs, completed by almost half the practices in Scotland, revealed glaring differences in the access people have to weight management clinics.

More than 25% of doctors claimed they did not use any specific criteria to identify which patients should receive extra support. Another quarter used a combination of patients' Body Mass Index and medical history while 40% used BMI alone.

Even then, patients had to be much fatter in some parts of the country before they got help.

While 18% of GPs said they would refer on patients with a Body Mass Index of 25, 16% waited until patients reached a BMI above 35.

Dr Stuart Scott, joint deputy chairman of the British Medical Association's Scottish GPs' committee, said GPs did not dictate these cut off points but the providers of the clinics.

He said: "What the GPs are doing depends on what is available in their local area. There isn't any consistency across Scotland."

Only two-thirds of the GPs questioned told SPHN they could send patients to exercise projects locally.

Most practices had leaflets to give adults about obesity, but 58% said they did not have any material to hand out about obesity in children.

The report suggests the lack of national targets, training for staff and ring-fenced funding could be behind the patchy picture as well as the lack of evidence showing which weight loss schemes work best.

It concludes: "A clear national political lead is required to drive forward the necessary changes in direction and to ensure that adequate resources are provided."

Dr Sarah Taylor, a public health director and chair of the working group behind the report, said: "I think overall we were encouraged (by the findings) because there are a lot of positive things going on, but there are some gaps."

Dr Colin Waine, chair of the National Obesity Forum, said: "A very important chronic disease is being subject to a postcode lottery. We have got to see that obesity is not a cosmetic issue. It is probably the biggest health problem that we are facing at this present time."

A diet and exercise programme costing £11.5m a year was promised in Scotland during the latest spending review.

A Scottish Government spokeswoman said: "We are supporting a wide range of actions that will contribute to people achieving and maintaining a healthy diet and healthy weight through implementation of our diet and physical activity strategies."