The way sport participation is measured should be changed, says the head of a group of 39 sporting governing bodies.
A public consultation into the future of UK sport has begun, amid falling participation figures.
But Adrian Christy, chairman of the National Governing Body CEO Forum, said the "whole picture" has not been presented.
"The current Active People Survey measurement presents a limited snapshot of participation," he said.
Whereas the latest figures for Sport Wales, Sport Scotland and Sport Northern Ireland show increases in participation levels, almost 250,000 people in England have stopped taking part in regular activity over the past six months.
A major overhaul of the way sport is funded across the UK will now be considered, with money potentially diverted away from national governing bodies (NGBs) and towards sports charities.
Christy, while welcoming the consultation as a "critical moment for sport", said "it is critical that NGBs remain at the heart of the sports system" to build on "the significant progress" achieved over the past decade.
The National Governing Body CEO Forum's membership includes 39 NGBs drawn from mass participation, Olympic, non-Olympic and disability sports.
"One area where we believe change is needed is in how participation is measured so that the whole picture is presented, not just part of it," said Christy, who is also CEO of Badminton England.
"The current Active People Survey measurement does not reflect participation among those under the age of 14, and does not make full use of modern technology in its sampling methods.
"It's counter-intuitive that one of the key groups that were meant to be inspired by London 2012 are not included in the APS."
Christy says sports such as hockey and handball have seen significant growth among young people in recent years - 40% more under-16s are playing hockey since 2012, for example.
He added: "In my own sport, we have witnessed incredible growth of young people playing in our National Schools Championships, from just over 300 players to 35,000. However the vast majority of these young players would not have been captured by the APS model."
|Is participation up or down?|
|Oct 2012-Oct 2013||Oct 2013-Oct 2014||Change|
|Source: Sport England's Active People Survey|