Alphabet's Google took a major step this year to end a European Union antitrust investigation with an offer to sell its advertising marketplace AdX but European publishers rejected the proposal as insufficient, two people with direct knowledge of the matter said on Wednesday.

Google's lucrative ad tech business attracted EU regulatory scrutiny last year following a complaint from the European Publishers Council. The European Commission subsequently charged Google with favoring its own advertising services, opening its fourth case against the world's most-popular search engine.

Google has never before offered to sell an asset in an antitrust case, according to three lawyers involved in antitrust cases who did not have permission to speak publicly. The company is on trial in the U.S., fighting claims by antitrust authorities who seek to make Google sell its Ad Manager product, which contains AdX and Google's publisher ad server, known as DFP.