An American watchdog group has threatened to sue global food chain McDonald's for what it termed promotion of child obesity through the Happy Meals scheme whereby the fast food giant gives away toys to children as part of its meal packages.

The Center for Science in the Public Interest says that the plastic promotions lure children into McDonald's restaurants where they are likely to order food that is high on calories, fat and salt.

The group served the fast food giant with a letter expressing intent to sue earlier this week. The Santa Clara county in the Silicon Valley had taken a lead to ban the toys from McDonald's Happy Meals promotional recently claiming that the toys which promoted the latest Shrek movie lured children to a high fat diet.

The letter, which is a legal requirement in the United States before lawsuits can be brought under the consumer protection statutes, was handed over to the fast food giant last Tuesday, reports the Los Angeles Times.

Stephen Gardner, the litigation director for the group, issued a statement where he described McDonald's as the as the proverbial stranger in the playground who hands out candy to children. McDonald's use of toys undercuts parental authority and exploits young children's developmental maturity,  he says.

However, the spokesman of McDonald's William Whitman disagreed with the statement and said the meals were of the right size for children and offered healthy choices. He also described toys as one part of the fun and family experience that the fast food giant promises.

McDonald's now offers the option of apple slices in lieu of French fries and provides children with the choice of ordering milk or juice with their Happy Means rather than an aerated drink. Whitman claimed that since the food chain started offering apple slices dipped in caramel sauce in 2008, more than 100 million orders had been placed for the meal.

The Santa Clara County had voted to ban the toy promotions from fast food meals and gave the chains up to three months to voluntarily comply before the ordinance became effective. The announcement, made in April, had won praise from nutrition experts while conservatives ridiculed it.