Tax deals involving Apple, Starbucks and Fiat and three EU countries are being investigated by the European Commission amid fears they broke state aid rules.
The probe focuses on agreements struck between Ireland and Apple; Starbucks and the Netherlands, and the financing arm of Italian automaker Fiat with Luxembourg.
Officials said with those deal they had lowered their tax bills by billions of euros, breaching European rules.
EU Competition Commissioner Joaquin Almunia declined to say how long the investigation would take, or speculate over the possible outcome.
“When public budgets are tight and citizens are asked to make efforts to deal the consequences of the crisis, it cannot be accepted that large multinationals do not pay their fair share in taxes,” said Almunia.
Commission officials stressed that it was the three countries which were under investigation, not the companies.
Dutch and Irish diplomats said their respective governments were convinced there had been no state aid.
A spokesman for Luxembourg’s finance ministry declined to comment.
The discussion over whether major multinationals pay enough tax has been a huge issue in recent years amid reports of how companies like Apple use complex structures to slash their tax bills.
Apple CEO Tim Cook was asked to appear before the US Senate in May 2013 to explain his company’s fiscal arrangements in Ireland, which Democratic Senator Carl Levin called, “the Holy Grail of tax avoidance”.
Euronews correspondent James Franey reports that the opening of this investigation marks the latest step in the global crackdown on corporate tax avoidance.
“If the Commission finds that these three member states have indeed broken the rules, the companies involved could be asked to pay back any ‘lost’ revenues,” he said.