The EU's highest court has ordered the European Commission to re-open an investigation into Ireland's travel tax.
The European Court of Justice has annulled a previous decision, meaning another inquiry will be held into whether the tax was a form of illegal state aid.
The case was taken by Ryanair, who complained about the tax to the European Commission in 2009.
The Commission dismissed that complaint in 2011 after a two-year inquiry.
The European Court of Justice has now found significant failures in this inquiry - which went on for too long, but which did not offer Ryanair the opportunity to make submissions.
The 'annulment' of the original decision will now leave the Commission with no option but to re-open its investigations.
The travel tax, introduced in Brian Lenihan's first Budget in October 2008, put a €10 levy on all passengers taking flights.
However, it also allowed for a €2 lower rate on flights where the destination was within 300 kilometres from Dublin Airport, irrespective of where in Ireland the flight departed.
It also exempted aircraft with fewer than 20 passengers - a combination of concessions which led Ryanair to complain that only flights by Aer Arann and Aer Lingus were able to benefit from it.
The tax was reformed to a €3 flat rate in 2011, and abolished entirely with effect from April 2014.
