About three-quarters of primary schools are not providing the recommended amount of physical education (PE) classes, according to the Education and Training Inspectorate (ETI).
Department of Education guidance is that pupils should have at least two hours of PE each week.
But the study found that 74% of primary schools were "unable" to do that.
The ETI report also said the pandemic had "impacted significantly" on PE in schools and children's activity.
Children were prohibited from playing outdoor sport for a number of months in early 2021 and PE in schools was also affected by lockdowns and limits on indoor gatherings.
