The Federal Trade Commission and 22 states sued Amazon on Monday, claiming the e-commerce giant secretly made advertising more expensive on its website for millions of companies.

In a lawsuit filed in a federal court in Washington, Amazon’s home state, the F.T.C. and the states said Amazon inflated the costs that brands and marketplace sellers had to pay to promote their products to consumers, including the prominent “Sponsored Product” ads that appear in search results on Amazon’s widely used website.

“Amazon has millions of advertising customers who were misled into paying significantly higher prices,” Andrew N. Ferguson, the chairman of the F.T.C., said in a statement. “These higher costs were largely passed on to American consumers.”

Amazon disputed the charges in a blog post. The company said the F.T.C.’s claim “fundamentally misunderstands how advertisers operate.” Amazon said its mechanisms led to more relevant ads, which created better value for brands and sellers, and that the lawsuit “complaint cites no evidence of consumer price increases.”

This is a developing story. Check back for updates.