A global survey says standards in reading, maths and science are falling.
Many Scottish Children Start School Obese
More than one in 10 children have started primary school overweight or obese in the last decade, according to new figures.
A total of 82,897 four to five-year-olds entered P1 carrying excess weight between 2005/06 and 2014/15, the Cancer Research UK study for World Obesity Day found.
The charity has warned obesity could become a "crippling burden on society and the NHS'' if action is not taken to tackle the problem, with such excess weight linked to 13 types of cancer.
As it launches its Scale Down Cancer campaign, it has called on the Scottish Government to do more to tackle obesity by restricting billboard advertising of unhealthy food and drink and reducing supermarket multi-buy discounts.
Professor Linda Bauld, Cancer Research UK's cancer prevention expert, based at the University of Stirling, said: "We should be concerned about the picture this paints for the health of the nation as we know that obese children are around five times more likely to become obese adults.
"Obesity is also linked to 13 types of cancer including bowel, breast and pancreatic. If left unchecked, we run the risk of obesity becoming a crippling burden on society and the NHS.
"The Scottish Government must do more to serve up a better future for our children and protect youngsters from being bombarded by junk food marketing on TV, as well as the barrage of supermarket multi-buy offers on sugar and fat-laden food and drinks.''
The charity found that over the past decade, around 15% of children in primary one each year were overweight or obese.
The nation has one of the heaviest populations in Europe with two in three adults overweight or obese, the charity said, while the cost of obesity to the NHS in Scotland is estimated at £600 million a year.
Nurse and mother-of-three Kirsty Thomasson, who weighed 13st and was a size 18 before slimming down, is backing the campaign.
She decided to lose weight after her trousers burst at the seams, and is now 9st 4lb and a size eight.
The nurse, from Inverurie in Aberdeenshire, feels she now has more energy for her children Cameron, four, three-year-old Arran and 17-month-old Lewis.
She said: "I think advertising does influence what you buy, that's why the junk food companies invest in it.
"If I saw a poster for a sugary snack, or an advert on TV, then it would definitely have encouraged me to put a particular biscuit or a fizzy drink in my shopping basket - especially if it was part of a 'buy one get one free' offer.
"I think we are all so busy, and we're bombarded with adverts for junk food all the time, that Scotland has become a fast-food nation.
"And, as a nurse, I see the awful consequences of what obesity can do to your health all the time.
"I think it's awful that so many children now are obese and it's about time we all took responsibility and did something about it.''
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