WASHINGTON 

Are the toys in your child’s Happy Meal making him fat? The Center for Science in the Public Interest says they are.

The Washington-based consumer advocacy group threatened to file a lawsuit against McDonald’s Tuesday, charging that the fast food chain ‘‘unfairly and deceptively’’ markets the toys to children.

‘‘McDonald’s marketing has the effect of conscripting America’s children into an unpaid drone army of word-of-mouth marketers, causing them to nag their parents to bring them to McDonald’s,’’ CSPI’s Stephen Gardner wrote to the heads of the chain in a letter announcing the lawsuit.

The center, which has filed dozens of lawsuits against food companies in recent years, is hoping the publicity and the threat of a lawsuit will force McDonald’s to negotiate with them on the issue. The group announced the lawsuit in the letter to McDonald’s 30 days before filing it with the hope that the company will agree to stop selling the toys before a suit is filed.

McDonald’s Vice President of Communications William Whitman said in a statement that the company ‘‘couldn’t disagree more’’ with CSPI’s assertion that their toys violate any laws. He said McDonald’s restaurants offer more variety than they ever have and Happy Meals are made smaller for kids.

‘‘We are proud of our Happy Meal which gives our customers wholesome food and toys of the highest quality and safety,’’ Whitman said. ‘‘Getting a toy is just one part of a fun, family experience at McDonald’s.’’

CSPI said the suit would be filed in state court. The center has not settled on a state yet, but the group believes the toys in Happy Meals violate state consumer protection laws in Massachusetts, Texas, the District of Columbia, New Jersey and California.

California’s Santa Clara County voted earlier this year to ban restaurants from giving away the toys and other freebies that often come with high-calorie meals aimed at kids.

McDonald’s has fought such criticism for years, and the company made a pledge in 2007 to advertise only two types of Happy Meals to children younger than 12: one with four Chicken McNuggets, apple dippers with caramel dip and low-fat white milk, or one with a hamburger, apple dippers and milk. They both meet the company-set requirement of less than 600 calories, and no more than 35 percent of calories from fat, 10 percent of calories from saturated fat or 35 percent total sugar by weight.

CSPI argues that even if those Happy Meals appear in advertisements, kids order the unhealthier meals most of the time.

The group is hoping its first lawsuit against the mega-chain will have a similar effect as its 2006 lawsuit against Kellogg that prompted the company to agree to a settlement raising the nutritional value of cereals and snacks it markets to children.

Still, some may accuse the group of extremism, arguing that it’s the parents’ responsibility to monitor what their children eat, not the restaurant’s.

Michael Jacobson, executive director of CSPI, says it’s the parents responsibility too, but he equates the toy giveaways to a door to door salesman coming to a family’s house every day and asking to privately speak with the children.

‘‘At some point parents get worn down,’’ Jacobson says. ‘‘They don’t always want to be saying no to their children. We feel like an awful lot of parents would be relieved if this one pressure was removed from them.’’

McDonald’s also came under fire over Happy Meals earlier this year when it recalled 12 million ‘‘Shrek’’ drinking glasses sold with the meals. The Consumer Product Safety Commission said the levels of the carcinogen cadmium in the glasses was too high.