Boris Johnson has called for schoolchildren to be given two hours of physical education a day to secure the sporting legacy of the London Olympics.
Amid criticism of the Government's decision to scrap the two-hours-a-week compulsory target, the Mayor of London said widening participation in sport after the Games was of "profound" importance.
He said efforts were already underway in London to increase children's participation in sport, adding: "I would like to see a much more thorough-going effort. I think we must build on the achievements of these Games."
Mr Johnson suggested Team London volunteers could be "mobilised" after the Games to "train kids up" in sports.
Earlier, shadow Olympics minister Tessa Jowell said all political parties should commit to a 10-year plan for sport to build on the public enthusiasm for London 2012.
The plan would focus on schools but Ms Jowell echoed cycling gold medal hero Sir Chris Hoy's call to ensure funding for elite training continues.
Sir Chris was speaking after Culture Secretary Jeremy Hunt said he could not promise to maintain the funding at its current level.
Ms Jowell said: "This has got to go all the way from the child coming into reception class in primary school, taking part in PE, right through to our extraordinary medallists.
"Chris Hoy is absolutely right, it has been the investment in elite training which has created stability for high performance training for those athletes.
"We have got to make sure that money continues."
Ms Jowell said there should be cross-party backing for a 10-year plan as schools with a good sports provision also do well in other areas.
"Schools that do a lot of sport tend to have better behaviour, less truancy, more children arriving on time. Schools that major on sport show better academic results, often from a very low base," she said.
Ms Jowell also called on the Government to bring back the commitment to two hours of compulsory PE in primary schools.
"Let's get that commitment back because PE in primary schools, not taught by teachers who are busy doing other things, but taught by teachers who are trained to teach PE to children, is absolutely essential."
Her comments followed David Cameron's call for a return to the "competitive ethos" in school sports.