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. Nevertheless, no matter what the outcome, the legal dispute will likely shed more light on the complicated nature of the online ad buying ecosystem.
SSPs enable publishers to sell their online ads through automated technologies, which is meant to be more efficient than getting a direct sales team to sell each slot individually.
The technology connects publishers' advertising inventory to multiple ad exchanges and demand-side platforms (DSPs - automated technology ad buyers use to target specific users across a range of websites), opening up their ads to a large range of potential buyers who bid in an online auction for the available slots in the miliseconds it takes for a web page to load.
Publishers pay SSPs a fee for their technology. Ad buyers also pay a fee to SSP in order to take part in the auction.
Publishers can demand in their contracts that the SSP discloses how much it is charging those buyers to participate in the auctions for their inventory. But many don't - or sometimes the wording in the contracts is not explicit - so lots of publishers simply receive the net ad rate (after the buyer fee has been taken off) that their SSP pays them, without knowing exactly how much the ad buyers actually paid in total. It's also tricky because SSPs charge different DSPs different fees, which can fluctuate according to circumstances.
Last year, The Guardian conducted a test where it bought its own ad inventory on open ad exchanges so it could get a sense of how much of the money put into the ad tech ecosystem made it back to the publisher.
In the worst case scenario, The Guardian found that for every £1 spent on its inventory, just 30p actually made it to The Guardian, as MediaTel reported in October.
Back then, The Guardian's chief revenue officer Hamish Nicklin said: "There are so many different players taking a little cut here, a little cut there - and sometimes a very big cut. A lot of the money that [advertisers] think they are giving to premium publishers is not actually getting to us."
Rubicon Project's share price has dropped almost 70% over the past year as the company reported a series of weak results after admitting it had been slow to jump on the latest trend in ad tech: header bidding.
Earlier this month, Rubicon Project hired ad tech veteran Michael Barrett as its CEO, replacing founder Frank Addante, who has moved to the chairman position. In an interview with Business Insider shortly after the management changes were announced, Addante explained the company is now "refocused and reinvigorated" around the focus of being "the largest independent global advertising exchange." Barrett also denied the rumor that the company is setting itself up for a sale.