From the article:

Google must face a £13.6bn lawsuit alleging it has too much power over the online advertising market, a court has ruled.

The case, brought by a group called Ad Tech Collective Action LLP, alleges the search giant behaved in an anti-competitive way which caused online publishers in the UK to lose money.

And the actual case at the UK's Competition Appeal Tribunal:

https://www.catribunal.org.uk/cases/15727722-15827723-ad-tech-collective-action-llp

The claims by Ad Tech Collective Action LLP are for loss and damage allegedly caused by the Proposed Defendants’ breach of statutory duty by their infringement of section 18 of the Competition Act 1998 and Article 102 of the Treaty on the Functioning of the European Union. The PCR seeks to recover damages to compensate UK-domiciled publishers and publisher partners, for alleged harm in the form of lower revenues caused by the Proposed Defendants' conduct in the ad tech sector.