A food watchdog group has sued General Mills for allegedly misleading customers with it Cheerios Protein cereal.

Cheerios Protein does not contain much more protein than regular Cheerios even though it's marketed that way, according to the suit filed by the Center for Science in the Public Interest (CSPI) in U.S. District Court in northern California.

"Rather than protein, the principal ingredient that distinguishes Cheerios Protein from Cheerios is sugar," CSPI said in the suit.

Golden Valley-based General Mills countered in a press statement said that Cheerios Protein contains 18 percent more protein by weight than original Cheerios..

"We don't normally respond to these publicity-seeking lawsuits from CSPI – but we do reject their comparisons. … Cheerios Protein is accurately labeled – and provides a good source of protein in every labeled serving," General Mills said in a press release.

Cheerios Protein was launched in 2014 to capitalize on the protein trend sweeping the food industry. The cereal has 7 grams of protein per serving while regular Cheerios has 3 grams of protein per serving, according to the Nutrition Facts panel printed on each box.

But CSPI attributes most of the difference due to a larger serving size listed on the box for Cheerios Protein: 55 grams versus 28 grams for regular Cheerios.

Mills is saying that even accounting for the serving size difference, Cheerios Protein still has 18 percent more protein than Cheerios.

As for sugar, regular Cheerios has 1 gram per serving, according to the Nutrition Fact box. Cheerios Protein has 16 to 17 grams, depending on the variety.