A leading charity is expanding its health programme in schools after research showed that NI children were among the least active in the UK.
The University College London study found that only 43% of seven-year-olds had the recommended one hour of exercise each day.
The research showed that girls fared worse than boys.
The charity, Chest Heart and Stroke, (NICHS) said something must be done to halt the increase in childhood obesity.
Chief executive Andrew Dougal said a time bomb was "being stored up for the future".
"In combination with too little exercise, too many children are eating poor diets which are high in fat, salt and sugar," Mr Dougal said.
"It's particularly worrying that the exercise survey was carried out among seven-year-olds, because children tend to become less, rather than more active as they become older."
He said childhood obesity in Northern Ireland had increased from 27% in 2010-2011 to 31% in 2011-2012 and was likely to rise further.
"To address the problem, we need to see co-operation between schools, parents, food producers, government and health charities," Mr Dougal said.
"That's one of the reasons we have expanded our health programmes in schools and plan to expand them further from the start of 2014."
The interactive programmes introduce primary and secondary school pupils to the need for a healthier lifestyle, and include tips on better eating, getting more exercise and giving up smoking.