The Guardian and Rubicon Project are set for a courtroom battle, after the publisher said it was preparing a lawsuit alleging the ad tech firm failed to disclose fees to advertisers.

According to reports, The Guardian believes Rubicon Project has imposed hidden costs on brands using its software to buy the newspaper’s digital inventory, and is preparing to file the lawsuit in the UK.

Guardian News & Media revealed its intention in a statement: “We can confirm that we have commenced proceedings against Rubicon Project for the recovery of non-disclosed buyer fees in relation of Guardian inventory. We cannot say anything further at this time.”

Rubicon Project has denied the claims, and argued that all its buyer fees are publicly disclosed in its SEC filings. It said it will “vigorously” contest in court what is considers to be a “contract dispute”.

It released the following response to The Guardian’s allegations: “We charge buyer fees for certain services we provide and have disclosed that fact publicly, including in our SEC filings, and in client contracts, including a contract we signed with Guardian over a year ago. We split our fees between sellers and buyers, reflecting the value we provide to both.

“Our marketplace fees on transactions support the considerable and compounding costs of performing an open auction – including our extensive brand protection and inventory quality screening, and malware protection. As we add new buyers and sellers onto the platform, the resulting impact is compounding infrastructure costs.

“Without buyer fees we would need to charge sellers more, and we think our approach is fair.”

In recent months Rubicon Project has announced a host of changes to its executive team, and plans to focus the business on the sell-side, rather than on ad buyers.

President Greg Raifman last month revealed he would be leaving the company, along with several other senior leaders. Former UK, Ireland and Nordics managing director James Brown has been appointed as EMEA managing director.