A national pet food giant is nipping at a smaller Connecticut-based competitor, alleging that the Wilton company is misleading consumers about the ingredients in its dog and cat foods.

In the lawsuit filed in U.S. District Court in St. Louis, Nestle Purina PetCare Co. accuses Blue Buffalo Co. Ltd., of false advertising, disparagement and unjust enrichment.

Blue Buffalo’s marketing pitch emphasizes that it uses only high-quality ingredients while foregoing chicken by-products, artificial flavors, corn, wheat or soy. Yet, St. Louis-based Purina says that independent testing reveals that Blue Buffalo uses “significant amounts” of those ingredients.

“We categorically deny all of these false allegations and will aggressively defend the integrity of our brand and our products,” Blue Buffalo founder and chairman, Bill Bishop, said in a prepared statement. “It is an easy thing to make unsubstantiated claims, put them in a lawsuit and then publish them all over the web to disparage and defame a company. It is quite another thing to prove those allegations.”

Bishop said the company does not use chicken or poultry by-products, ground corn or artificial preservatives. “We will prove these and other matter in the court with good reliable evidence, and we look forward to disproving the voodoo science that Nestle Purina relied on to support their outrageous allegations,” Bishop said.

Last year, $21.5 billion was spent on pet food in the United States, according to the American Pet Products Association, with more expensive foods comprising a rising share of the market. Sales of upscale brands rose 68 percent from 2002 to 2012, compared with 19 percent for mid-priced brands and 8 percent for economy brands, according to Euromonitor International, a global market research company. Nestle Purina has been part of the trend, marketing a Beyond line that’s “made with natural ingredients.”

Blue Buffalo also advertises that its pet foods contain natural ingredients, which are touted as being healthier for pets and less likely to cause allergic reactions. But in its lawsuit, Purina claims that testing by an independent laboratory showed that several Blue Buffalo products contained significant percentages of poultry byproduct meal and corn. The suit also says several products promoted as “grain-free” contain rice hulls.

Purina’s lawsuit states that Blue Buffalo has spent “ten of millions of dollars” to purchase advertising and to hire a “small army of in-store marketers.”

“Despite Blue Buffalo’s massive marketing barrage, Purina has discovered that Blue Buffalo — and not the `big name’ pet food manufacturers Blue Buffalo routinely criticizes in its advertising — is concealing the truth about the ingredients in its products,” the lawsuit states.

The lawsuit asks the court to force Blue Buffalo to run corrective advertising to show that its products contain chicken byproducts, and to pay damages to Nestle Purina “for all gains, profits, savings and advantages obtained by Blue Buffalo as a result of its false advertising and unfair competition.”

This is the second time this year that Blue Buffalo’s sales tactics have been portrayed in a negative light. The Hartford Courant reports that in March, an investigation by the Better Business Bureau recommended that Blue Buffalo change its advertisements which show pet owners’ being fooled by competing brands.