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, saying in a post on social media it was very unfair and threatening to launch an investigation over European tech practices that could lead to tariffs.
As I have said before, my Administration will NOT allow these discriminatory actions to stand, he wrote.
The European Union must stop this practice against American Companies, IMMEDIATELY!
Trump has repeatedly criticised the bloc's fines and enforcement actions against US tech firms in recent months, though the US government has brought its own lawsuits over Google's monopoly of the online ad market.
Earlier this week, the Commission denied reports it had delayed the announcement of Google's fine amid tensions over trade relations between the EU and the US.
In the Commission's decision on Friday, Google was accused of self-preferencing its own technology above others.
As part of its findings, it said Google had intentionally boosted its own advertising exchange, AdX, over competing exchanges where ads are bought and sold in real-time.
Competitors and publishers faced higher costs and reduced revenues as a result, which may have led to increased consumer prices.
The regulator has ordered the company to cease such practices, along with paying the nearly €3bn penalty.
Third time rules broken
The fine is among the largest handed down by the Commission to tech firms for competition rule breaches so far.
In 2018, Google was fined €4.34bn (£3.9bn) for using its Android operating system to cement itself as the dominant player in that market.
Teresa Ribera, executive vice president of the Commission, noted that previous findings of Google's anti-competitive conduct influenced the higher fine.
We increased Google's fine since this is the third time Google breaks the rules of the game, she stated.
Ms. Ribera warned that Google must detail how it will alter its practices within 60 days, or else the Commission will consider imposing its own solutions.
At this stage, the only way for Google to completely address its conflict of interest is through a structural remedy, such as divesting part of its ad tech business, she added.
