Tensions between the EU’s legal order and the international investment law regime are not exclusive to the Brexit era, but they certainly gained momentum in the aftermath of this referendum. By incautiously declaring that the UK will remain a party to the Unified Patent System regardless of Brexit, the British government arguably shaped (il)legitimate expectations on the part of investors who aimed at exploiting their intellectual property rights in the UK while benefitting from the judicial protection of the forthcoming Unified Patent Court as much as of the European institutions (and market) as a whole. Indeed, not only the System itself will undergo a process of major rebalancing after London’s departure from the EU, but more importantly, the UK will most probably be unable to retain its membership in the System after the actual delivery of Brexit. These complications trigger a wide spectrum of fundamental dilemmas investing the definition and scope of concepts such as unilateral declaration, indirect expropriation, reasonable expectation, estoppel, and public policy exception, under both EU law and international investment law. It is therefore essential to explore these intersections as to anticipate possible scenarios in the event of both domestic court and international arbitral claims lodged by patent investors pre- and post-Brexit, having due regard for competition concerns on the side of the EU, yet referring to recent Canadian case law which opened the gate to investor-State claims in the field of intellectual property.