Until 2002, the venue for European Council summits was the member state that held the rotating Presidency of the Council of the European Union. However, European leaders agreed during ratification of the Nice Treaty to forego this arrangement at such a time as the total membership of the European Union surpassed 18 member states. An advanced implementation of this agreement occurred in 2002, with certain states agreeing to waive their right to host meetings, favouring Brussels as the location. Following the growth of the EU to 25 member states, with the 2004 enlargement, all subsequent official summits of the European Council have been in Brussels, with the exception of punctuated ad hoc meetings, such as the 2017 informal European Council in Malta. The logistical, environmental, financial and security arrangements of hosting large summits are usually cited as the primary factors in the decision by EU leaders to move towards a permanent seat for the European Council. Additionally, some scholars argue that the move, when coupled with the formalisation of the European Council in the Lisbon Treaty, represents an institutionalisation of an ad hoc EU organ that had its origins in Luxembourg compromise, with national leaders reasserting their dominance as the EU's "supreme political authority".