European Union antitrust regulators have asked Ireland to provide details of its tax arrangements with Apple (AAPL.O) in 1990 and 2007, warning that they could amount to illegal state aid which may be recovered from the U.S. company.

In a letter published on Tuesday, European Commissioner Joaquin Almunia told Ireland it must provide the details of the tax arrangements, which in his preliminary view could constitute illegal state aid and therefore be recoverable from Apple.

"The Commission's preliminary view is that the tax ruling of 1990 (effectively agreed in 1991) and of 2007 in favor of the Apple group constitute state aid," Almunia wrote.