Apple embarks on an epic court battle with the EU on Tuesday, fighting the commission's landmark order that the iPhone-maker reimburse Ireland 13 billion euros (14 billion U.S. dollars) in back taxes.

Lawyers for the world's biggest company will face EU officials in a Luxembourg court, challenging a decision that CEO Tim Cook slammed at the time as "total political crap".

The European Commission's conclusion was delivered in August 2016 by Competition Commissioner Margrethe Vestager, a shock decision that put Europe at the forefront of an emerging effort to rein in the power of U.S. big tech.

The two days of hearings on Tuesday and Wednesday will take place at the EU's lower General Court, where judges will give their judgment no earlier than 2020.

Any appeal would then go the EU's highest court, the European Court of Justice, for a final decision that could land as late as 2021.

The EU accuses Apple of parking untaxed revenue earned in Europe, Africa, the Middle East and India, in Ireland, which has emerged as a European hub for big tech and global pharma giants.

This privilege allegedly gave Apple an advantage over other companies, allowing it to avoid taxes between 2003 and 2014 of around 13 billion euros which, according to Brussels, constituted illegal "state aid" by Ireland.

Apple fiercely denies the tax bill. The U.S. government also insists the order by Brussels constitutes a major breach of international tax law.

"The European Commission has tried to rewrite Apple's history in Europe, to ignore Ireland's tax laws and, in doing so, to disrupt the international tax system," Tim Cook said in an open letter in 2016.

The group insists that it is in the United States, where the company invests in research and development and thus creates wealth, that it must pay taxes on the revenue in question.

This became possible after a major tax overhaul in the U.S. at the end of 2017 that allowed Apple to repatriate profits made abroad. Apple has promised to pay Washington a tax bill of 37 billion U.S. dollars, in addition to the taxes already paid in the United States.

'Strong case'

The California-based giant is supported in its fight by Ireland which has also appealed, refusing to be singled out as a tax haven.

"We will present a very strong case," promised Irish Finance Minister Paschal Donohoe on Friday.

The two days of hearings are taking place in a tense trade context between the EU and the United States where President Donald Trump accuses Europeans of deliberately attacking American technology giants.

The EU's Competition supremo, Vestager, is in particular accused by the U.S. president of "hating" the U.S. He has slammed her as the "tax lady" because of the investigations and heavy fines imposed on U.S. groups such as Google.

Pending the conclusion of the case, Apple has blocked the funds in an escrow account: a total of 14.3 billion euros, after interest.

The group, which has been present in Ireland since the 1980s, employs around 6,000 people in Cork, the country's second-largest city.

The first signs of how the Apple case may finish will come as early as September 24 when the General Court will rule on whether the EU was right to demand unpaid taxes from Starbucks and a unit of Fiat Chrysler.