Europe's antitrust regulators are set to pounce on Google with formal charges Wednesday in their biggest anti-competition case since taking on Microsoft a decade ago, reports say.

European Union commissioner Margrethe Vestager has decided to pursue a case to challenge the American tech giant's practice of highlighting its own services in its search results at the expense of smaller rivals, The Wall Street Journal reports.

Fines could theoretically exceed $6 billion, the Journal reports.

"It means that [EU regulators] don’t want to settle [the] case, they want to adopt an infringement decision" that could lead to fines and injunctions, University College London professor Ioannis Lianos told the newspaper.

The New York Times reports the decision to pursue the case is the latest twist in the lengthy investigation into Google’s activities in Europe, where it holds about a 90 percent share in the region’s search market.

The world’s biggest search engine has come under mounting pressure in Europe over competition and privacy issues, Bloomberg reports, and EU lawmakers last year called for regulators to consider splitting up the company if they couldn't complete their antitrust probe.

Besides possible fines, Google also might be forced to give smaller competitors like Yelp a lot more prominence in search queries, The Times notes.

Microsoft was hit with a more than $1 billion penalty from the European Union for failing to comply with a 2004 antitrust ruling.

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