BRUSSELS (AFP) -- The European Union said it will unveil on Tuesday details of investigations into sweetheart tax deals for Apple amid reports it will accuse the U.S. tech giant of getting illegal state aid.

The probe into Apple's tax affairs in Ireland, where it has for years enjoyed an effective rate of less than 2.0 percent, found the schemes gave the company an unfair competitive advantage, the Financial Times and Wall Street Journal reported.

The EU launched an investigation in June into tax deals negotiated by Apple, Fiat and Starbucks with Ireland, Luxembourg and the Netherlands, the latest in a series of clashes between Brussels and multinational firms.

Apple and other giants including Amazon have come under intense pressure from politicians and campaigners over their tax dealings, with critics saying the arrangements allow companies to move billions in earnings from higher taxed countries to lower taxed ones.

The European Commission, the executive branch of the 28-nation EU, will take the next formal step against Apple by publishing on Tuesday the full reasons behind June's decision to open the investigation, a spokesman said.

It will also publish details of the decision to open the investigation into Italian automaker Fiat's tax deals on Tuesday, but not on the Starbucks decision.