Abstract While trade mark rights themselves constitute fundamental rights within the right to property, they may also collide with the fundamental rights of third parties. The latter is particularly evident where third parties try to justify their unauthorized use of a trade mark on the basis of their own fundamental rights. The collision between the fundamental rights–protected interests of the trade mark proprietor and the third parties become all the more apparent when it comes to the protection of trade marks with a reputation. Trade marks with a reputation are particularly exposed to use by third parties due to their high level of appreciation and recognition among the public and therefore deserve a special protection. However, this special protection must not get out of hand and must be restricted wherever the fundamental rights interests of third parties outweigh those of the trade mark proprietor. As a result, a balance must be struck between the colliding fundamental rights interests. All this raises the question where such a balancing of interests could take place in the context of the European Union (EU) Trade Mark Regulation 2017/1001 (EUTMR) and Directive on the European Union Trade Mark (EUTMD). And once the ‘where’ has been resolved, a question still remains as to how an appropriate balance can be achieved or rather what criteria need to be taken into account when balancing these interests. To answer these questions, existing case law of the Court of Justice of the European Union (CJEU) provides only limited to no substantive guidance. It is therefore not surprising that a Belgian national court requested the CJEU to clarify precisely these questions at EU level in 2023 (IKEA, C-298/23). In this case, the main role is played by the negative condition ‘without due cause’, which arises from the EUTMR and EUTMD provisions protecting trade marks with a reputation and which stipulates that the trade mark proprietor can only enforce the special protection if the third party is unable to establish that the use of the trade mark takes place with ‘due cause’. It is therefore the task of the CJEU to clarify whether the required balance of fundamental rights interests can be achieved within the framework of the undefined legal concept of ‘due cause’ and, if so, to specify the criteria by which such a balance can be achieved. This article sets out and discusses how the CJEU should solve this task and concludes that the concept of ‘due cause’ must be interpreted broadly in order to provide a flexible counterweight to the far-reaching protection of trade marks with a reputation. Hence, the concept of ‘due cause’ can be understood as a safeguarding mechanism for the fundamental rights of third parties, whereby various criteria must be taken into account in order to achieve a fair balance with the fundamental rights–protected interests of the trade mark proprietor.