Judgment

The Court (Grand Chamber) hereby:

1. Orders that Cases C-542/18 RX-II and C-543/18 RX-II be joined for the purposes of the judgment;

2. Declares that the judgments of the General Court of the European Union (Appeal Chamber) of 19 July 2018, Simpson v Council (T-646/16 P, not published, EU:T:2018:493) and HG v Commission (T-693/16 P, not published, EU:T:2018:492) affect the unity and consistency of EU law in so far as the General Court, as court of appeal, held that the composition of the panel of judges of the European Union Civil Service Tribunal which had delivered the order of 24 June 2016, Simpson v Council (F-142/11 RENV, EU:F:2016:136) and the judgment of 19 July 2016, HG v Commission (F-149/15, EU:F:2016:155), respectively, had been irregular, on the basis of an irregularity affecting the procedure for the appointment of one of the members of that panel of judges, leading to a breach of the principle of the lawful judge, laid down in the first sentence of the second paragraph of Article 47 of the Charter of Fundamental Rights of the European Union, and set aside those decisions;

3. Sets aside those judgments;

4. Refers the cases back to the General Court of the European Union;

5. Orders Mr Erik Simpson, the Council of the European Union, HG, the European Commission and the Bulgarian Government to bear their own costs in relation to the review proceedings.