Such a judgment from the Court of Justice of the European Union (ECJ) could boost UPS, the world's largest package delivery company, in its lawsuit against the European Commission for 1.7 billion euros (1.51 billion pounds) in damages.

The Commission in its 2013 decision said UPS had not offered sufficient concessions to address its concerns that the 5.2-billion-euro ($6.1 billion) deal would lead to higher prices.

UPS subsequently challenged the EU rejection of its bid for TNT, which was later acquired by Fedex.

The General Court, Europe's second-highest, said the EU competition enforcer had wrongly blocked the deal and it annulled the decision on a procedural irregularity. The Commission then appealed to the Court of Justice of the European Union (ECJ).

ECJ Advocate General Juliane Kokott suggested the top court should follow suit.

"The Court of Justice should confirm that the Commission's decision prohibiting the acquisition of TNT Express by UPS should be annulled due to a procedural error," Kokott said in a non-binding opinion.

The ECJ follows such opinions in the majority of cases.

"As the General Court correctly held, the Commission infringed UPS's rights of defence by making material changes to the econometric model on which it relied during the administrative procedure without informing UPS or giving the undertaking an opportunity to submit observations," Kokott said.

UPS welcomed the opinion.

"If the full court follows the advocate general, then this will confirm that the European Commission was wrong to block our bid for TNT," the company said in an email.

The case is C-265/17 P, European Commission versus United Parcel Services.

(Reporting by Foo Yun Chee; editing by Philip Blenkinsop and Emelia Sithole-Matarise)

By Foo Yun Chee