A two-year investigation into whether Apple’s tax deal with Irish authorities gave the iPhone maker an unfair advantage will take longer because of the large amount of data involved, according to the European Commissioner for Competition.

The European Commission accused Ireland in 2014 of dodging international tax rules by letting Apple shelter profits worth tens of billions of dollars from tax collectors in return for maintaining jobs.

Commissioner Margrethe Vestager said she had asked Ireland for more details, which in turn raised new questions that required a response from the authorities and from Apple.

"The first priority is the quality of the case work. It is very difficult to make predictions as to when the case will be ready for a decision," Vestager told a European Parliament hearing. Both Ireland and Apple have denied any special tax deal.

Vestager also said she had asked British tax authorities for details on their back-tax deal with Google following a complaint from the Scottish National Party.

The British government hailed the £130 million settlement, announced in January, as a major success.

Google has said it paid all due taxes.