BRUSSELS—European Union antitrust regulators are preparing to step up their investigations into Google Inc. on several fronts, including revisiting a proposed settlement over its search-engine practices that has met with unprecedented opposition.

The European Commission is likely to revise some terms of a settlement announced in February, aimed at dealing with concerns that the company abuses its dominance of online searches in Europe, a person with knowledge of the situation said Tuesday. That means a final decision on the...