The company must now end the conduct within 90 days or face penalty payments of up to fiveper cent of the average daily worldwide turnover of Alphabet, Google's parent company.

Commissioner Margrethe Vestager, in charge of competition policy, said: "Google has come up with many innovative products and services that have made a difference to our lives. That's a good thing."

But Google's strategy for its comparison shopping service wasn't just about attracting customers by making its product better than those of its rivals. Instead, Google abused its market dominance as a search engine by promoting its own comparison shopping service in its search results, and demoting those of competitors.

"What Google has done is illegal under EU antitrust rules. It denied other companies the chance to compete on the merits and to innovate. And most importantly, it denied European consumers a genuine choice of services and the full benefits of innovation."

In 2004, Google entered the separate market of comparison shopping in Europe, with a product that was initially called "Froogle", re-named "Google Product Search" in 2008 and since 2013 has been called "Google Shopping". It allows consumers to compare products and prices online and find deals from online retailers of all types, including online shops of manufacturers, platforms (such as Amazon and eBay), and other re-sellers.

When Google entered comparison shopping markets with Froogle, there were already a number of established players. Contemporary evidence from Google shows that the company was aware that Froogle's market performance was relatively poor  - one internal document from 2006 stated "Froogle simply doesn't work".

Comparison shopping services rely to a large extent on traffic to be competitive. More traffic leads to more clicks and generates revenue. Furthermore, more traffic also attracts more retailers that want to list their products with a comparison shopping service. Given Google's dominance in general internet search, its search engine is an important source of traffic for comparison shopping services.

Vestager said that from 2008, Google began to implement in European markets a fundamental change in strategy to push its comparison shopping service. This strategy relied on Google's dominance in general internet search, instead of competition on the merits in comparison shopping markets:

Google has systematically given prominent placement to its own comparison shopping service: when a consumer enters a query into the Google search engine in relation to which Google's comparison shopping service wants to show results, these are displayed at or near the top of the search results.

Vestager added that Google has demoted rival comparison shopping services in its search results: rival comparison shopping services appear in Google's search results on the basis of Google's generic search algorithms. Google has included a number of criteria in these algorithms, as a result of which rival comparison shopping services are demoted. Evidence shows that even the most highly ranked rival service appears on average only on page four of Google's search results, and others appear even further down. Google's own comparison shopping service is not subject to Google's generic search algorithms, including such demotions.

As a result, Google's comparison shopping service is much more visible to consumers in Google's search results, whilst rival comparison shopping services are much less visible.

The evidence shows that consumers click far more often on results that are more visible, i.e. the results appearing higher up in Google's search results. Even on a desktop, the ten highest-ranking generic search results on page 1 together generally receive approximately 95 percent  of all clicks on generic search results (with the top result receiving about 35 percent of all the clicks). The first result on page 2 of Google's generic search results receives only about one percent of all clicks. This cannot just be explained by the fact that the first result is more relevant, because evidence also shows that moving the first result to the third rank leads to a reduction in the number of clicks by about 50 percent. The effects on mobile devices are even more pronounced given the much smaller screen size.

This means that by giving prominent placement only to its own comparison shopping service and by demoting competitors, Google has given its own comparison shopping service a significant advantage compared to rivals.

Vestager said that Google's practices amount to an abuse of Google's dominant position in general internet search by stifling competition in comparison shopping markets.

"Market dominance is, as such, not illegal under EU antitrust rules. However, dominant companies have a special responsibility not to abuse their powerful market position by restricting competition, either in the market where they are dominant or in separate markets,"  she said.