Google has been fined €2.95bn (£2.5bn) by the EU for allegedly abusing its power in the ad tech sector - the technology which determines which adverts should be placed online and where.
The European Commission said on Friday the tech giant had breached competition laws by favouring its own products for displaying online ads, to the detriment of rivals.
It comes amid increased scrutiny by regulators worldwide over the tech giant's empire in online search and advertising.
Google told the BBC the Commission's decision was wrong and it would appeal.
It imposes an unjustified fine and requires changes that will hurt thousands of European businesses by making it harder for them to make money, said Lee-Anne Mulholland, global head of regulatory affairs at Google.
There's nothing anti-competitive in providing services for ad buyers and sellers, and there are more alternatives to our services than ever before.
US President Donald Trump also attacked the decision, calling it very unfair and threatening to launch an investigation into European tech practices that could lead to tariffs.
In the Commission's decision on Friday, it accused Google of self-preferencing its own technology over others, and noted that the company had intentionally boosted its own advertising exchange, AdX, over competing exchanges.
As part of its findings, Google was directed to end such practices and comply with a nearly €3bn penalty.
The Commission’s fine is one of the largest it has handed down to tech companies for competition rule violations to date. It also highlights a growing global concern over the power of tech giants and their impact on market competition.
Teresa Ribera, executive vice president of the Commission, indicated that unless Google detailed how it would change its practices within 60 days, further actions would be taken, potentially requiring Google to restructure its ad tech business.
