The GuardianThe Guardian is preparing to file a lawsuit against ad tech company Rubicon Project, alleging the ad tech vendor did not disclose fees it levied on advertisers looking to buy the newspaper's online ad inventory, sources told Business Insider. The Guardian is due to file its legal papers at the UK High Court's Chancery Division, Business Insider understands. A Guardian spokesperson confirmed the matter with Business Insider: "We can confirm that we have commenced proceedings against Rubicon Project for the recovery of non-disclosed buyer fees in relation of Guardian inventory." Rubicon Project did not immediately respond to a request for comment. Business Insider understands the amount The Guardian is looking to recuperate from the supply-side platform (SSP) spreads back over a number of years, but is only in the single-digit millions.