Google has been fined 2.42bn euros ($2.7bn; £2.1bn) by the European Commission after it ruled the company had abused its power by promoting its own shopping comparison service at the top of search results.
The amount is the regulator’s largest penalty to date against a company accused of distorting the market.
The US company may decide to appeal.
If it fails to change the way it operates the service within the three-month deadline, it could be forced to make payments of 5% of its parent company Alphabet’s average daily worldwide earnings.
Google had previously suggested that Amazon and eBay had more influence over the public’s spending habits and that the commission’s views “failed to fit the reality of how most people shop online”.
However, the decision could set a precedent that determines how the EU’s civil service handles related complaints about the prominence Google gives to its own maps, flight price results and local business listings within its search tools.
Google Shopping displays relevant products’ images and prices alongside the names of shops they are available from and review scores, if available.
The details are labelled as being “sponsored”, reflecting the fact that, unlike normal search results, they only include items that sellers have paid to appear.
On smartphones, the facility typically dominates “above-the-fold” content, meaning users might not see any traditional links unless they scroll down.
Google also benefits from the fact the Shopping service adverts are more visual than its text-based ads.
One recent study suggested Shopping accounts for 74% of all retail-related ads clicked on within Google Search results. However, the BBC understands Google’s own data indicates the true figure is smaller.