The usual notice period is two weeks. It has long contended deals made by European Union -member Ireland with multinational companies such as Apple constitute illegal state aid.
News of the decision made its way through the back corridors of power last week, prompting an unusual attack from the US Treasury in which it accused the EC of a power grab and argued that it was acting like "a supranational tax authority that reviews member states".
Although the decision itself does not specify the precise amount of money in play, Vestager will set out an estimate when the findings are released on Tuesday, according to people briefed on her plans.
The decision, which is likely to generate a fierce backlash from Dublin, Silicon Valley and Washington, will be Brussels' biggest attack on alleged tax avoidance by USA multinationals to date and is likely to trigger a quick appeal from both Apple and the Irish government.
Last month, Facebook (FB) was slapped with a tax bill from the Internal Revenue Service for between $3 billion and $5 billion for another Dublin-involved scheme. "I hope that we get a fair hearing". "If we don't, then we would obviously appeal it".
Ireland's low corporate tax rate has been a cornerstone of economic policy for 20 years, drawing investors from major multinational companies whose staff account for nearly one in 10 workers in Ireland.
On the other hand, the Irish finance minister, Michael Noonan has said the government will appeal any negative finding, the report notes.
In its argument, Apple says that instead of paying corporation taxes overseas, it should do so when it chooses to repatriate the funds to the USA as it develops its products there only.
Starbucks Corp. (SBUX) and Fiat have both previously been ordered by the European Commission to pay back taxes, and Amazon.com (AMZN) is under investigation.
The full ruling against Ireland is expected to be published early on Tuesday morning.