The EU's antitrust chief said she will take a tough stance against competition problems arising from Big Data - like the huge amounts of user information companies including Facebook (FB - Get Report) and Alphabet's Google (GOOG - Get Report) use for advertising, marketing and other purposes.

"It's true that we shouldn't be suspicious of every company which holds a valuable set of data," said EU Competition Commissioner Margrethe Vestager Friday in Copenhagen.

"But we do need to keep a close eye on whether companies control unique data, which no one else can get hold of, and can use it to shut their rivals out of the market," she said, adding that "if we do find that companies are undermining competition, we won't hesitate to take action."

So far, national competition and privacy watchdogs in the 28-country bloc rather than the EC in Brussels have taken the lead on reining in Big Data.

Germay's Bundeskartellamt, or Federal Cartel Office, in March opened an investigation into whether Facebook may have forced users to accept privacy terms that run counter to data protection rules.

And in France, Google is fighting an order to remove certain search results worldwide in an appeal to that country's top tribunal.

Vestager has on several occasions also hinted at more antitrust intervention from Brussels on tie-ups involving Big Data assets.

As an example, she mentioned the EC's 2008 review of Google's $3.1 billion purchase of online ad provider DoubleClick, which the regulator cleared under her predecessor after finding that DoubleClick's ad data was not unique.

"So the problem for competition isn't just that one company holds a lot of data," she said. "The problem comes if that data really is unique and can't be duplicated by anyone else."

Separately on Friday, Vestager again defended the EC's recent ruling against Apple (AAPL - Get Report) and the use of EU state aid rules to go after corporate tax dodgers.

Speaking at the Copenhagen Business School, she underscored that the decision was based on facts and that her job "is enforcing the rules and removing the distortion created by the advantage that a company got by paying much less tax."