The European Commission has warned Ireland it may investigate more than just Apple as part of a probe into the country's tax practices.
The EU is investigating whether Ireland, Luxembourg and the Netherlands have attracted investment and jobs by helping big companies avoid tax in other countries, including EU members.
EU commissioners are worried that by acting as tax havens Apple and Google have been slashing their tax bills at the expensive of other European nations.
Ireland is being seen as being far too too lenient in rulings it gave to Apple and which helped the company shield tens of billions of dollars in profit from tax. The commission has asked Dublin for information on the rulings it gave the iPhone maker.
Commission officials said they believed other companies had also benefited from generous treatment and want Ireland to change its approach so companies cannot shift so much profit through Ireland into tax havens.
Apple has denied receiving special treatment from the Irish authorities. Ireland, the Netherlands and Luxembourg said any tax rulings they give are in line with international practices.
But pushed by France and Germany, Brussels is keen to clamp down on what it sees as unfair tax competition across the bloc.
"Member states' tax incentives should never be used to lure profits away from where they should rightfully be taxed," EU tax commissioner Algirdas Semetas told a meeting of EU finance ministers in Luxembourg on Friday, according to the text of his speech published by the EU. "We must verify that the principles of fair play are not being undermined."
If the Commission can prove Ireland agreed tax treatments which diverge from international rules, it could deem any corporate tax savings to be a form of subsidy which must be halted or even repaid.