SBUX Starbucks Corporation

BRUSSELS—European Union regulators have deepened a high-profile investigation into alleged sweetheart tax deals after all EU countries agreed to cooperate with the inquiry, the bloc's antitrust chief Margrethe Vestager said.

The tax investigations, a political priority for the EU's executive arm, has already embroiled four multinational companies, including Apple Inc. and Amazon Inc., and could lead to demands for large back-tax payments.

"All member states are now cooperating with our inquiries," Ms. Vestager told European lawmakers at a hearing on Thursday. "This has allowed us to take the inquiries to a deeper level."

At a hearing in May, Ms. Vestager had complained of difficulties in obtaining information from some national governments, notably the Czech Republic, Estonia and Poland. Luxembourg also initially resisted the EU's requests for tax documents and had fought the case in court, before relenting in December.

With governments now cooperating, Ms. Vestager said she had asked countries for a "large number" of tax agreements with individual companies, to assess whether they breached EU law.

Tax rulings are used to confirm the size of companies' future tax bills, but the EU has said it suspects some of the agreements may have granted certain firms an advantage over others, which would be illegal under EU law.

"We have asked each member state to provide us with 10 to 12 concrete tax rulings to give an idea of how tax rulings are used," Ms. Vestager said.

The EU's tax investigation has been repeatedly broadened in recent months in a sign of its high political priority—despite concerns around the role of the European Commission's president Jean-Claude Juncker in supporting several such deals when he was Luxembourg's Prime Minister.

At a time of austerity across much of the EU, governments are eager to crack down on tax avoidance to shore up their finances and demonstrate to taxpayers that wealthy multinationals are paying their fair share of taxes.

The EU said in June that it would ask 15 national governments to provide further details of "a substantial number of individual tax rulings," after broadening its inquiry in December to all EU countries from an initial group of six.

The investigation has sparked detailed probes into tax deals struck by four multinationals—Apple in Ireland, Amazon and Fiat SpA in Luxembourg and Starbucks Corp. in the Netherlands.

Regulators had initially pledged to decide by the end of June whether those four deals violated EU law. But Ms. Vestager admitted in May that her agency would miss that deadline, and declined to give a new one.

"We will close our cases as soon as we are ready to do so," the commissioner told lawmakers on Thursday. She said she wouldn't hesitate to open more detailed investigations if she found evidence that individual tax deals had violated EU law.

Write to Tom Fairless at tom.fairless@wsj.com

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(END) Dow Jones Newswires

September 17, 2015 08:25 ET (12:25 GMT)

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