Teachers need to do more to get the hardcore of non-exercising pupils in England back in the school gym, Schools Secretary Ed Balls has said.
Mr Balls warned girls could be put off by "embarrassing" sports kit
Alternative activities such as Frisbee and yoga could be offered alongside traditional sports, he said.
The government says a target to get 85% of youngsters doing two hours of sport a week by 2008 has been met already.
But ministers are still warning that half the UK population may be obese in 25 years' time.
Prime Minister Gordon Brown is also highlighting his £100m campaign to get schools offering children five hours of sport a week.
In a BBC interview, he stopped short of backing calls for children to be weighed and measured at school.
But he did say parents needed to know at an early stage if there was a problem and pledged to do everything in his power to ensure that young people were growing up fit and healthy.
"We have got to look at what's going to be the best for the child," he added.
Mr Balls hailed what he described as a "quiet revolution" in school sport in recent years.
He said: "Sport - like school food - is important to improve children's lives and reduce childhood obesity.
"There is now a wide variety of sporting opportunities available, including both the traditional and the less familiar."
But he called on schools to concentrate on those children who were not taking advantage of existing sports opportunities.
He said: "We know that girls, especially as they get older, often stop taking part in sport."
He told the Guardian that the way in which schools provided sports had a big influence on participation rates.
Schools needed to offer a wider range of sporting activities such as Frisbee and yoga, alongside the more traditional school sports.
He added: "If the kit is awful or embarrassing it's much more likely the kids will forget to bring it."
The official 2006-07 School Sport Survey found 86% of pupils in 21,700 maintained schools in England were doing at least two hours of physical education or school sport a week.
Traditional sports of football, gymnastics, dance, athletics and cricket remained the most popular.
But the variety of sports was on the increase with an average of 21.7 different ones being offered.
These ranged from mountaineering to the Asian team sport, kabbadi, while 31% of schools were offering golf, 42% cycling and 29% canoeing.
Schools are also getting better at identifying sporting excellence, with the share of children registered as gifted and talented up from 3% in 2003-04 to 7% in 2006-07.
The Liberal Democrats said the obligatory two hours of PE a week was a fraction of the amount of time children spent watching television, playing computer games or surfing the Internet.