The EU's top court rules Wednesday on whether Islamist movement Hamas should be taken off the bloc's terror black list, a move that would likely spark anger in Israel and the United States.
Washington and Israel were outraged in December 2014 when a lower European court said Hamas should be dropped because the EU had made the decision based on information from the media and internet.
Brussels subsequently appealed that decision and the European Court of Justice in Luxembourg, the top judicial authority in the 28-nation European Union, will now have the final say on the matter.
A senior lawyer to the ECJ has already said in an official legal opinion delivered last September that Hamas should not have been included on the list because procedural mistakes invalidated the EU decision.
The court will also rule Wednesday on a similar case involving Sri Lankan rebel group the Liberation Tigers of Tamil Eelam (LTTE).
If Hamas is dropped, already tense EU-Israel relations are like to be tested again. Israel regularly berates the European Union for being soft on terrorism and bluntly rejects EU criticism of its Jewish settlements policy.
Last week, Israeli Prime Minister Benjamin Netanyahu denounced the EU's political demands on Israel as "absolutely crazy."
The European Union imposed travel bans and asset freezes on Hamas, which controls the Gaza Strip, after the September 11, 2001 Al-Qaeda terror attacks on New York and Washington.
Hamas opposed the sanctions from the start, arguing that it is a legally elected government and therefore has the right to conduct military operations against Israel.
The original 2014 ruling by the EU's General Court annulled the EU sanctions listing on procedural grounds.
It said that rather than establishing independently that Hamas was a terrorist organisation, the European Council of EU member states had instead relied on publicly available information.
The European Council in turn appealed that finding, believing the General Court "was wrong in its assessment of the way in which the Council relied on information in the public domain".
ECJ Advocate General Eleanor Sharpston said in September that the EU could not "rely on facts and evidence found in press articles and information from the internet, rather than in decisions of competent authorities, to support a decision to maintain a listing".
Given that "some of the reasons advanced could not justify the decision to maintain the listing," the General Court was correct to dismiss the EU appeal when it could find no other sufficient reasons for their being listed.
Accordingly, the ECJ "should annul the measures... on procedural grounds," Sharpston said.
She said in her September opinion that the EU decision to sanction the LTTE was similarly flawed and should be annulled.
Advocates general of the ECJ are regularly called on to give their view before it makes a final ruling. The court often but not always follows them.
The EU maintains an active sanctions policy, targeting individuals, groups and states, including several other Palestinian entities.