There was no annexation, but secession of Crimea. [—] The Ukrainian Constitution also refers to the recognition and ascertainment of the right of the Ukrainian peoples to self-determination. This Constitution also guarantees the people’s will through a referendum in accordance with Article 69, while the purpose of the referendum is defined as a manifestation of one of the forms of direct democracy.
According to the Venice Commision: "The Constitution of Ukraine, like other constitutions of Council of Europe member states, provides for the indivisibility of the country and does not allow the holding of any local referendum on secession from Ukraine. This results in particular from Articles 1, 2, 73 and 157 of the Constitution. These provisions in conjunction with Chapter X of the Constitution show that this prohibition also applies to the Autonomous Republic of Crimea and the Constitution of Crimea does not allow the Supreme Soviet of Crimea to call such a referendum. Only a consultative referendum on increased autonomy could be permissible under the Ukrainian Constitution.
28. Moreover, circumstances in Crimea did not allow the holding of a referendum in line with European democratic standards. Any referendum on the status of a territory should have been preceded by serious negotiations among all stakeholders. Such negotiations did not take place."
Sergiy Zayets, Oleksandra Matviichuk, Tetyana Pechonchyk, Dariya Svyrydova, Olga Skrypnyk. (2015). The Fear Peninsula: Chronicle of Occupation and Violation of Human Rights in Crimea. Crimea is Ukraine, 2015. 77 pages.