TORONTO - Ontario is facing a "serious problem'' with overweight children and must do more to protect them by banning advertising directed at kids, NDP critic Rosario Marchese said.

He plans to introduce a bill Monday amending the Consumer Protection Act to prohibit commercial television advertising for food or drink that is directed at a child under the age of 13.

"We believe that marketers know when marketing works and it does affect dietary choices that children make,'' he said.

"We know that corporations hire a lot of psychologists to market adequately to every category of age groups that one can think of. So we know that it affects kids.''

Research shows that one in four Canadian children between the ages of four and 17 is obese, and it's no wonder when children are being bombarded by television ads promoting sugar-packed soft drinks and other products, Marchese said.

"What kids see on television is high in calories and low in nutrients,'' he added. "That's generally what kids watch on television.''

Toronto's medical officer of health has long recommended a ban and Quebec has banned all advertising aimed at children since 1982, Marchese said.

The ban would also prohibit ads that promote healthy foods like yogurt and cereal, but Marchese said he's confident parents will be able to help their children make better dietary choices.

"The general point is that children are very vulnerable and it's very difficult for them to make intellectual distinctions between... good and bad,'' he said.

"Because we consider children to be a vulnerable group, our view is that we should prohibit commercial advertising altogether, so that we don't have to worry about distinguishing between this and that. We leave that to parents.''

Ontario's Liberal government introduced legislation in December that bans foods containing trans fats from school cafeterias and vending machines, but the NDP has complained that was a watered down effort.

Chocolate bars, potato chips and soft drinks have already been banned from Ontario's elementary schools, but the Liberals say the legislation would enshrine that policy in law and eventually expand the junk food ban to include high schools.

Trans fats, often found in french fries and other fast-food cafeteria staples, are being targeted around the world by advocates of healthy eating, who say the processed oil contributes to a host of health problems, including childhood obesity.

Manitoba's NDP government has proposed to ban the sale of any food containing trans fats in the province's schools, while Prince Edward Island and Alberta have a voluntary ban on trans fats.

The federal NDP have been pushing for a Canada-wide trans fat ban, which has been recommended by an all-party parliamentary task force.