88. At the European regional level, the Explanatory Report to the ECN, with regard to arbitrary deprivation of nationality, provides in Article 4(c) that “the deprivation must in general be foreseeable, proportional and prescribed by law for it not to be arbitrary.161 89. In the context of Article 8 ECHR, an interference or restriction of this right, for instance by (arbitrary) deprivation of nationality, should be based on a legal norm that is clear, accessible (meaning that it is published or otherwise possible for an individual to have knowledge thereof) and foreseeable (meaning that it enables a person to act in accordance with the law).162 90. Deprivation of nationality “must have a firm legal basis , should not be interpreted extensively or applied by analogy and “deprivation-provisions must be predictable .163 Furthermore, the 2020 UNHCR Guidelines No. state that “laws that permit deprivation of nationality on the grounds of terrorism should be publicly available and precise enough to enable individuals to understand the scope of impermissible conduct. 164 The use of administrative measures, which include deprivation of nationality, must be prescribed by law. This law should “be clear, predictable and accessible to the public .165 91. As with all laws dealing with criminal offences, the power to deprive a person of his nationality must not be enacted or applied with retroactive effect. This is indicated by the provision in Article 8(4) of the 1961 Convention according to the Tunis Conclusions: “[…] a legal provision regarding loss or deprivation of nationality may not be enacted or applied with retroactivity, nor may a provision regarding the acquisition of nationality be repealed or restricted with retroactivity. To establish whether a person acquired or had a nationality withdrawn on account of certain acts or circumstances, the legislation which was in force at the moment these acts occurred is to be applied. Where a new ground for loss or deprivation of nationality is introduced in national law, the State must include a transitional provision to avoid an individual losing his or her nationality due to acts or facts which would not have resulted in loss or deprivation of nationality before the introduction of the new ground.”166 Similarly: “The principle of ‘tempus regit factum’: To establish whether a person acquired or had a nationality withdrawn through certain acts or facts, the legislation has to be applied which was in force at the moment when these acts or facts took place. […]”167 161 Explanatory Report to the ECN, para 36. This follows from case law of the European Court of Human Rights, with regard to Article 8 for instance in Silver and Others v United Kingdom (1983), Application no. 7136/75 et al, para 86-88; Lebois v Bulgaria (2017), Application no. 67482/14, para 66-67. 163 ILEC Guidelines (2015), Section I.1, I.3, I.7. 164 para 65. 165 Glion Recommendations (2019), p. 8. 166 UNHCR, ’Tunis Conclusions‘ , para -17. 167 ILEC Guidelines (2015) I.6. 162 53