In his opinion on case C 284/16: Slovak Republic v. Achmea BV, Advocate General Melchior Wathelet concluded on the compliance of intra-EU BITs with EU law:
“Articles 18 TFEU, 267 TFEU and 344 TFEU must be interpreted as not precluding the application of an investor/State dispute settlement mechanism established by means of a bilateral investment agreement concluded before the accession of one of the Contracting States to the European Union and providing that an investor from one Contracting State may, in the case of a dispute relating to investments in the other Contracting State, bring proceedings against the latter State before an arbitral tribunal.”
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For recent information about the opinion on case, please see the link or please see on the epravo website in Czech.