The European Commission has ordered Ireland to recoup 13 billion euro from tech giant Apple over a sweetheart tax deal.
The judgement follows a three-year probe by EU Competition Commissioner Margrethe Vestager's office into the tech giant's structures and "back-room" deals dating back to 1991.
In 2014, Brussels warned the Dublin government it had doubts over the compatibility of the arrangements with EU internal market regulations.
The European Commission said it had concerns that tax calculations amounted to a sweetheart deal by under-estimating taxable profit on Apple products like iPhones and iPads, creating an unfair advantage.
The exact cost of the unpaid taxes is not expected to be set out by the Commissioner on Tuesday but she is expected to outline her findings and a process for calculating the bill.
Whatever the decision, both Apple and the Irish Government are likely to challenge it in the European courts.
The case is one of the most high-profile in the fight to redraw boundaries on aggressive tax avoidance, a fight which has put the EU at odds with the US government.
Apple has had a base in Ireland since 1980 and employs around 5,500 people in the country, with its biggest operations in Cork.
The basis of the case against Ireland and Apple, both of which repeatedly denied breaching state aid rules, is that the multinational secured a tax advantage which was not available to other companies.
The inquiry examined how much tax Apple paid in Ireland, which has a much-maligned corporation tax rate of 12.5% for business profits.
Ms Vestager's ruling is also coming just a week before Apple's biggest product launch of the year, with the iPhone 7 and a new version of the Apple Watch to be unveiled in San Francisco.
Her office's investigations have also targeted aggressive tax planning by Starbucks and Fiat, both of which are appealing against rulings ordering them to pay back taxes to the Netherlands and Luxembourg.
Ireland's Department of Finance declined to comment in advance of the Commissioner's announcements.
But government sources in Dublin warned they were steeling themselves for an "adverse finding" and one added: "We could be talking about figures in the billions."
Ahead of the Commissioner's announcement, Apple outlined the scale of its operations across Europe, where it has 22,000 employees.
It also claimed to have helped create more than 1.4 million jobs since its App Store launched eight years ago, and said it spent 11 billion euro (£9.4 billion) with more than 4,500 European suppliers in 2013.
Apple also has plans for a major data centre near Athenry, Co Galway, in Ireland and another in Denmark.
Chief executive Tim Cook told the Washington Post earlier this month: "I hope that we get a fair hearing. If we don't, then we would obviously appeal it."
