National provisions derogating from a harmonisation measure
Since the main objective of a measure adopted according to Article 114 TFEU is precisely to remove disparities between the relevant rules and regulations of the Member States, every national provision which deviates from a harmonisation measure is a potential problem. At the same time there is a strong interest, at least among some Member States, not to be wholly deprived of the ability to pursue a higher level of environmental and health protection than the EU does through its harmonising measures.
To address this problem the SEA introduced what became known as the ‘environmental guarantee’, according to which:
If, after the adoption of a harmonisation measure by the Council acting by a qualified majority, a Member State deems it necessary to apply national provisions on grounds of major needs referred to in Article 36, or relating to protection of the environment or the working environment, it shall notify the Commission of these provisions.
The Commission shall confirm the provisions involved after having verified that they are not a means of arbitrary discrimination or a disguised restriction on trade between Member States.31
However, it gave rise to competing interpretations on a number of issues— including if it applied in respect of measures adopted by a State before it joined the EC and whether a Member State could rely on it even it if had not voted against the harmonising measure in the Council—some of which were eventually addressed by the Court of Justice.32
The Treaty of Amsterdam introduced amendments which made it clear that it was not only the introduction of new provisions but also the maintenance of existing ones that should be notified and assessed by the Commission. A certain time limit within which the Commission has to make a decision was also introduced.
The preconditions for maintaining national provisions are now set out in Article 114(4), which reads:
If, after the adoption of a harmonisation measure by the European Parliament and the Council, by the Council or by the Commission, a Member State deems it necessary to 
3i Art l00a(4) EEC.
maintain national provisions on grounds of major needs referred to in Article 36, or relating to the protection of the environment or the working environment, it shall notify the Commission of these provisions as well as the grounds for maintaining them.
Unlike in the case of the original wording, there is now no doubt that a Member State may rely on this provision regardless of what decision-making procedure applied for the adoption of the harmonisation measure and regardless of how it voted. It should also be noted that the provision applies also with respect to measures adopted by the Commission. Since a legislative act, according to Article 290 TFEU, may only delegate to the Commission the power to adopt acts to supplement or amend certain ‘non-essential elements’ of the legislative act, measures adopted by the Commission shall, in principle, not generally affect the level of protection pursued. But in practice, seemingly detailed and technical measures, like the inclusion of a substance in an Annex to a directive or regulation, or its removal therefrom, can be controversial and have quite significant effects on the level of protection pursued.
When a Member State has notified provisions which it deems it necessary to maintain, the Commission shall assess the compatibility of these provisions with the conditions in Article 114(6), including whether or not they are a means of arbitrary discrimination or a disguised restriction on trade. These will be further discussed after the additional requirements have been presented; these must be met when a Member State wishes to introduce new provisions in an area subject to a harmonisation measure.
-  2 See, eg, Case C-319/97 Kortas ECLI:EU:C:1999:272.