The article dwells upon the key factors which shaped the approach of Viscount Castlereagh, the British Secretary of State for Foreign Affairs (1812 – 1822), with respect to the Polish question in the run-up to the Congress of Vienna 1814 – 1815, the evolution of the diplomat's attitude towards this problem, as well as the diplomatic struggle between the British Foreign Secretary and his European counterparts over the post-war organization of the territories inhabited by the Poles. The research task consists in the attempt to analyse the conceptual framework of the Foreign Office's attitude towards the post-war territorial settlement of the Polish lands, as well as Castlereagh's personality factor, that was of significant importance in the course of his diplomatic struggle over the Polish cause with the Great Powers' representatives, especially with Alexander the Great. The article's novelty consists in the attempt to place the Foreign Secretary's position at the centre of the analysis of a crucial issue considered at the Congress of Vienna, and to touch upon the influence of Castlereagh's political background and some domestic and foreign factors that influenced his position. As a result, the author draws the conclusion that the final settlement of the Polish question was successful for Castlereagh and beneficial for the United Kingdom in spite of the difference between the final decisions and the demands of the Foreign Secretary that were expressed on the eve of the Congress. Besides, there is a continuity between Castlereagh's views on the European territorial settlement and the "just equilibrium" concept, set forth by W. Pitt the Younger: a necessary condition for its realisation was the prevention of accession of Polish lands to only one Great Power. Moreover, the author emphasises the unprecedented for then British Foreign Secretaries involvement in the continental problems, which coincided with the decisive European events.