A highly reactive polyurethane powder coating composition, which contains uretdione groups that are hardenable at low temperatures, contains A) at least one uretdione-containing powder coating hardener based on aliphatic, (cyclo)aliphatic or cycloaliphatic polyisocyanates and hydroxyl-containing compounds, the hardener having a melting point of from 40 to 130.degree. C., a free NCO content of less than 5% by weight, and a uretdione content of 6-18% by weight; B) at least one hydroxyl-containing polymer having a melting point of from 40 to 130.degree. C., and an OH number of between 20 and 200 mg KOH/g; C) 0.001-3% by weight of at least one tetraalkyl ammonium carboxylate catalyst, based on a total amount of components A) and B). Components A) and B) are present in a ratio so that for each hydroxyl group of component B) there is from 0.3 to 1 uretdione group of component A).