Primary teachers get too little training in how to teach physical education, say government inspectors, with some students receiving as little as seven-and-a-half hours' tuition.
Most training is practical and of high quality but time constraints mean that its value is limited, according to an Office for Standards in Education report.
The findings confirmed sports campaigners' suspicions and re-awakened training providers' worries that the national curriculum - even in its slimmed-down form - is too bulky to cover adequately in a single year's postgraduate certificate in education course.
There have already been complaints that art and music are being squeezed in the primary training syllabus.
OFSTED inspected PE at 20 higher-education institutions and on one school-centred initial teacher-training course. It found trainees on the one-year postgraduate course received an average of 23 hours in PE training. Undergraduates on three or four-year courses were given 32 hours.
Some students on both types of course got seven-and-a-half hours.
An OFSTED spokesman said: "This is a very short time if trainees are to learn how to teach effectively all six components of national curriculum PE." The curriculum includes games, dance, gymnastics, athletics, swimming and outdoor and adventurous activities - with the last three most likely to be dropped.
Sports campaigners warned that the current situation - where PE is being squeezed out of school timetables in the drive to raise numeracy and literacy standards - was likely to worsen.
They say sport is important for children's development because it helps them develop co-ordination and motor skills, language, numeracy and concepts like time and space, which all aid academic achievement.