Boris Johnson has insisted that protecting school playing fields is vital for the health of young people today.
The mayor of London said the policy within the capital was to preserve existing playing fields, except when replacements are to be provided elsewhere.
He said: "We don't believe in selling off playing fields and I think if you look at what we do in London, we have very clear rules that you can't sell off green space, you can't sell off playing fields.
"What you can do if you do need to expand a school - and there's huge pressure on school places - you can do that provided you increase the availability of playing fields, so you've got to have new provision of playing fields elsewhere."
Mr Johnson, who recently called for schoolchildren to do two hours of sport every day, was speaking after the government was forced to apologise for issuing misleading figures about the sell-off of school playing fields.
Ministers have approved the sale of 30 school pitches, nine more than the Department for Education previously admitted to signing off.
The department blamed officials for providing education secretary Michael Gove with incorrect information.
The mayor added that London's schools do not have sufficient outside space.
"Playing fields are absolutely vital for the health and wellbeing of young Londoners.
"Many schools don't have enough, many parts of inner London simply don't have enough playing space, and it's vital that we conserve what we have."
Mr Johnson made the comments at the Sea Life London Aquarium, where he was promoting a range of summer discounts on attractions, hotels, restaurants and theatres in the capital.
A DfE spokesman said: "We are sorry to say that the secretary of state was provided with incorrect information about how many playing fields were disposed of since May 2010.
"The figures presented to the secretary of state, and published by the department, related to applications received between May 2010 and June 2012.
"Those figures should have included requests received by the previous government and then approved by the coalition."
Most approval decisions are taken by junior ministers after "careful consideration", the spokesman said.
Shadow education secretary Stephen Twigg said: "The fact that Michael Gove has ignored the advice of independent experts and ploughed ahead with selling off school playing fields shows he is shamefully out of touch.
"He also appears to have failed to disclose at least another 10 school playing field sell-offs when responding to a freedom of information request. This is misleading and incompetent, at the very least.
"Michael Gove must now come clean and explain what appears to be a secret programme to sell off school playing fields."