A report presented to Hillingdon Council also suggested more than a fifth of 4-5 year-olds in the borough can be classed as such.
The 'concerning' figures look set to spark a deeper investigation.
The Children, Young People and Learning Policy Overview Committee presented the figures at a council cabinet meeting.
Its report stated: “Data presented to the committee during the review indicated some concerning levels of childhood obesity in Hillingdon. 21% of 4-5 year olds in Hillingdon were obese.
“Although levels in Hillingdon compared reasonably well against London and national averages, what was of more concern was the increased levels of obesity amongst the year 6 cohort when compared to 4-5 year olds, with 34% of 10-11 year olds in Hillingdon being overweight or obese.
“Concerns were expressed during the review about the significant differences in childhood obesity levels between wards in Hillingdon.
“While noting that the issue of obesity is already being addressed through an obesity strategy, the committee recommends that the concerns raised during the review in relation to levels of child obesity in Hillingdon be passed to the relevant council committee and officers for them to consider whether further investigation or review would be appropriate.”
After the meeting on 18 February, Graham Hawkes, chief executive of Healthwatch Hillingdon, told Hillingdon is Here he believes a lack of education lies behind the problem.
He said: “Schools spend a lot of their time and effort in making their grades good rather than actually educating children on wellbeing.
“The schools have got targets to reach, and the health and wellbeing of their students is not their number one priority. The number one priority for them is reaching the criteria for their exam results.
“We have to go and make sure that children are coming out of primary school and nursery school, to actually say, ‘mum, dad, you need to stop smoking and we need to eat better’, because otherwise we're going to die at 60 rather than 80.”
Mr Hawkes also said that parents are unwilling to let their children walk to school, due to safety concerns and the fact that both parents in a household often have jobs. This busy family life, he believes, can also lead to an over-reliance on fast food.
He said: “It’s a 21st Century lifestyle problem. The whole world is our oyster now - with all those foods that come with it - and we don’t have the exercise to burn it off.”
Councillor Nick Denys, Vice-Chairman of the Children, Young People and Learning Policy Overview Committee, agreed that a lack of exercise in children is a nationwide, rather than Hillingdon problem.
He said: “In the old days, kids used to go out to the park and play football, when a lot more often they now spend time sitting indoors playing computer games.
“We’ve put money into leisure centres and swimming pools and the cost of using those. We’ve insured that they’re at least 10% cheaper than any of the neighbouring boroughs so as to keep the cost of using them manageable.”
During the review, concerns were also raised regarding Hillingdon’s “significant differences in childhood obesity levels between wards”.
While the report does not go into specifics, it was suggested that southern wards such as Yiewsley and West Drayton fared worse than wards in the north of Hillingdon, like Ickenham, Ruislip and Harefield.
Mr Hawkes said that a “completely different make up of residents in different communities” plays a part in this.
He said:“We have a definite north/south divide, except for a few pockets. If you live in the north you can live for 7 years more than you would in the south.
“There are less children in the north than there are in the south. There is more deprivation and poverty in the south, and that all comes in to it.”
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