A British campaign against obesity says children under the age of five — even those who can't walk yet — should exercise every day.

In its first such guidelines for children that young, the British Health Department says children under five who can walk should be physically active for at least three hours a day.

Officials also said parents should reduce the amount of time such children spend being sedentary while watching television or strapped in a stroller.

The three hours of activity should be spread throughout the day. Officials said a child's recommended level of exercise could likely be met simply through play, but also by including activity such as walking to school.

For babies who can't walk yet, the health department said physical activity should be encouraged from birth. Infants can benefit from playing on their stomachs or having swimming sessions with their parents.

Children's individual physical and mental abilities should be considered when interpreting the advice, the health department said.

"It's vital that parents introduce children to fun and physically active pastimes to help prevent them becoming obese children, who are likely to become obese adults at risk of heart disease, diabetes and some cancers," Maura Gillespie, head of policy and advocacy at the British Heart Foundation, said in a statement.

Nearly a quarter of British adults are obese, and experts estimate that by 2050 about 90 per cent of adults will be heavy.

According to a 2008 health survey that used devices to measure how much people actually exercised, officials found only about five per cent of Britons meet the government's minimum physical activity advice — about 150 minutes of moderate to vigorous exercise every week, including some every day.

For children aged five to 18, Britain recommends at least one hour of exercise, but that should include intensive activities to strengthen muscles and bones.

In the U.S., the Centers for Disease Control and Prevention advises children and teenagers to get about one hour or more of physical activity every day.

According to the Institute of Medicine, an independent organization in Washington, D.C., toddlers should get at least 15 minutes of exercise for every hour they spend in child care.

In Canada, children aged five to 11 and youth aged 12 to 17 are encouraged to accumulate at least 60 minutes of moderate- to vigorous-intensity physical activity daily, such as walking quickly, skating, bike riding and skateboarding.

The Canadian Society for Exercise Physiology said Monday it is working on Canadian physical activity and sedentary behaviour guidelines for children under the age of five that will be released in spring 2012.