Khan called on all local authorities in London to follow the lead of Southwark, south London, which has announced all 68 primary schools in the borough will ban sugary drinks.
London has one of the highest rates of childhood obesity in Europe with almost 40 per cent of children in year 6 classed as "overweight" or "obese".
The mayor created London’s Child Obesity Taskforce in 2018 to help develop ideas that would reduce obesity levels and close health inequality gaps, with some of the highest levels of obesity being in the most deprived areas of the capital.
Making water freely available to pupils and introducing water-only schools – where just water and low-fat milk are allowed as drinks – was one of the recommendations of the taskforce.
In addition to calling on more boroughs to adopt the policy, Khan is also writing to Ofsted to ask that they encourage schools and pre-schools to promote healthy diets, water and physical activity.
He is also inviting representatives from water companies and the advertising industry to form a new partnership group to consider how to encourage children to drink more water.
Khan said: “It’s high time we faced up to the reality that our city has a child obesity problem and I’m clear that we all have a role to play in helping Londoners make healthy choices.
"Making water freely available and introducing measures like ‘water-only’ in schools are just some of the simple things we can do to not only tackle childhood obesity, but also improve pupil behaviour and learning. I’m urging all schools in London to follow Southwark’s example and adopt the policy to help improve the life chances of all young Londoners.”
Paul Lindley, chair of London’s Child Obesity Taskforce, said: “We need to do all that we can to make sure London is a city in which children can grow up healthier and thrive.
"Research shows that increasing drinking water in schools is a great way to help young Londoners start to make healthier choices and improve their own wellbeing and academic outcomes. London’s child obesity taskforce is taking bold steps to tackle child obesity which will have a detrimental impact on young Londoners now and in the future. I am delighted all primary schools in Southwark are adopting our water-only policy. I hope all schools across the capital follow their lead.”
Southwark Council said its data indicates that black, Asian and ethnic minority (BAME) children run the greatest risk of obesity ahead of children of other ethnic backgrounds.
In an attempt to motivate behavioural change and perception, Southwark primary schools will offer water only. Children in nursery will have reduced-fat milk in classes. The milk offered will include lactose free, soya and other alternatives. Skimmed or semi-skimmed milk will also be available for nursery classes.
Evelyn Akoto, lead member for community safety and public health at Southwark Council, said: "In Southwark, odds are stacked against some of our children based on economic circumstances and social preferences for sugary drinks for refreshment or hydration. In these circumstances, sugary drinks have all but replaced water.
“In February, all Southwark schools were sent details of the new water-only policy to allow ample time for teachers and school staff to work with pupils to transition to water only. This policy will be mandatory for all primary schools in Southwark (and nursery classes) from the start of summer term in April 2020.
"Schools will put in place processes to advise parents about the new policy and to offer guidance as needed. Promoting drinking water and banning sweetened drinks should kick-start making healthier choices and improving children’s health from nursery onwards.”