British news giant The Guardian is preparing to sue ad tech provider Rubicon Project, claiming the company did not disclose fees it levied on advertisers looking to buy the newspaper’s online inventory, according to reports.

Business Insider understands The Guardian is due to file its legal papers at the UK High Court’s Chancery Division.

A spokesperson for The Guardian confirmed with Business Insider that the news publisher “has commenced proceedings against Rubicon Project for the recovery of non-disclosed buyer fees in relation of Guardian inventory”.

The amount The Guardian is looking to recuperate from the supply-side ad platform is in the single-digit millions, and spreads back over a number of years, according to Business Insider.