The decision was made by the Steering Board of the Peace Implementation Council at a session in Brussels. The appointment is to be formally approved by the United Nations Security Council and the Council of the European Union.
Lajcak, 43, will be the sixth High Representative since the office was established under the 1995 Dayton peace agreement that ended the war in Bosnia and Herzegovina. He is to assume the post on June 30, succeeding Christian Schwarz-Schilling of Germany.
The Steering Council earlier decided that the High Representative would retain the existing broad powers, which include the possibility of imposing laws and dismissing elected officials, at least by the summer of 2008.
Lajcak is currently the Director-General for Political Affairs in the Slovak Ministry of Foreign Affairs. He previously served as Ambassador to Belgrade, and was also the EU special envoy for Montenegro's independence referendum.