Representatives of the Polish government officially took over the civilian administration of the southern part of East Prussia on 23 May 1945. Subsequently Polish expatriates from Polish lands annexed by the Soviet Union as well as Ukrainians and Lemkos from southern Poland, expelled in Operation Vistula in 1947, were settled in the southern part of East Prussia, now the Polish Warmian-Masurian Voivodeship. In 1950 the Olsztyn Voivodeship counted 689,000 inhabitants, 22.6% of them coming from areas annexed by the Soviet Union, 10% Ukrainians, and 18.5% of them pre-war inhabitants. The remaining pre-war population was treated as Germanized Poles and a policy of re-Polonization was pursued throughout the country Most of these "Autochthones" chose to emigrate to West Germany from the 1950s through 1970s (between 1970 and 1988 55,227 persons from Warmia and Masuria moved to Western Germany). Local toponyms were Polonised by the Polish Commission for the Determination of Place Names.

In what year did the Polish government officially take over the administration in East Prussia?