Apple Inc. has set up a court battle with European Union competition watchdogs who ordered Ireland to claw back a record 13 billion euros ($13.6 billion) in unpaid taxes from the iPhone maker.

The U.S. tech giant said Monday it formally appealed the EU’s August decision to the EU’s General Court in Luxembourg, as the European Commission and Ireland separately published details of their own arguments in the case.

In an order that reverberated across the Atlantic, the EU slapped Apple with the multibillion-euro bill, saying Ireland granted unfair deals that reduced the company’s effective corporate tax rate. The U.S. Treasury said the EU was making itself a "supra-national tax authority" that could threaten global tax reform efforts.

Bloomberg News

The EU "took unilateral action and retroactively changed the rules, disregarding decades of Irish tax law, U.S. tax law, as well as global consensus on tax policy, that everyone has relied on," Apple said in a statement after filing its court appeal.

Ireland has already asked the court to strike down the EU’s decision from August. In a separate filing on Monday, the Brussels-based commission published details of its two-year probe, attacking the way Irish authorities taxed profits attributed to two of Apple’s Irish units.

The EU also took aim at Irish tax practices, saying they were "too inconsistent" to set firm rules for profit allocation.

The Apple investigation -- and others probing arrangements for Inc. and McDonald’s Corp. in Luxembourg -- are the cornerstone of an EU attack on national practices it claims help multinational companies avoid tax.

EU Competition Commissioner Margrethe Vestager has repeatedly argued that special tax treatment harms companies that don’t get such advantages.

According to Monday’s EU filing, regulators shrugged off claims that they weren’t fair to Ireland and Apple during the probe. They argued they always respected their procedural rights and the subject matter of the investigation -- profit allocation methods used in tax rulings -- never changed.

Ireland had "ample opportunity" to express its views and "made use of that opportunity on multiple occasions," the EU said. While Apple only had the right to submit observations, it "was given and has effectively made use of the opportunity to submit" views to regulators on numerous occasions, according to the text of the EU decision.

The EU’s competition arm said it doubted that the methods used to produce taxable profit for Apple units complied with the so-called arms-length principle for transfers between parts of a company.

Ireland said the EU exceeded its powers by ruling on its tax jurisdiction, according to an Irish finance ministry statement on Monday.

Ireland also challenges the EU’s determination that Apple allocated almost all its European sales profits to what the EU said were “head offices” not subject to tax by virtue of their tax status in Ireland. Apple argues the units, Apple Sales International and Apple Operations Europe, were never exempt from tax -- and that under the law they are subject to deferred U.S. tax.

“These branches carried out routine functions, but all important decisions within ASI and AOE were made in the U.S., and the profits deriving from these decisions were not properly attributable to the Irish branches of ASI and AOE,” the Irish finance ministry said.

The court appeals filed by Apple and Ireland follow those already pending by Luxembourg, the Netherlands and Belgium concerning tax arrangements the nations granted to units of other multinationals, including Starbucks Corp.

Though Apple will have to pay its tax bill within weeks, the money will be held in escrow. A final ruling from the EU courts may take several years.