1944 Icelandic constitutional referendum
A constitutional referendum was held in Iceland between 20 and 23 May 1944. The 1918 Danish–Icelandic Act of Union declared Iceland to be a sovereign state separate from Denmark, but maintained the two countries in a personal union, with the king of Denmark also being the king of Iceland.
Source: Wikipedia — 1944 Icelandic constitutional referendum (CC BY-SA 4.0)