[nomadking]

Link to EU Parliament briefing (pdf)

The right of a Member State to withdraw from the European Union was introduced for
the first time with the Lisbon Treaty. Prior to that, the question of the right of
withdrawal was highly controversial. Some authors had argued for the application of
customary international law (clausula rebus sic stantibus, also established in Article 62
of the Vienna Convention on the Law of the Treaties providing for unilateral withdrawal
from international treaties) within the EU framework. Moreover, several constitutional
courts saw a unilateral right to withdraw from the EU as necessary in order to preserve
national sovereignty.
...
The inclusion of a right to withdraw from the Union in the Draft Constitutional Treaty
was based on the premise that such a withdrawal would have been permissible anyway
through application of the general principles of international law.