The article deals with a little-studied episode of the reign of Empress Elizaveta Petrovna, related to the secret correspondence of Catherine II, then still a Grand Duchess, with the British Ambassador in St. Petersburg Charles Hanbury Williams (1756–1757). The correspondence published in Russia (1909) and the United Kingdom (1928) is a unique source in terms of volume and significance. It provides an opportunity to show the hidden side of diplomacy in the times of large-scale geopolitical shifts on the eve of and during the Diplomatic Revolution of 1756 and the Seven Years’ War. Moreover, the correspondence with the British Ambassador reveals Catherine's power ambitions during the August and October crises of the Empress's chronic illness, the political background of her relationship with S.A. Ponyatovsky. The subject of the research is a new source found in the State Archive of the Russian Federation – “A Note on Sir Hanbury Williams, His Relationship with Catherine II, and the Politics of His Time”. The “Note” consists of five chapters. The material of the article is grouped into three blocks characterizing the main directions of Sir Williams' diplomatic activity in St. Petersburg, his participation in the intrigues at the court of Elizaveta Petrovna, and his role in the romantic affair between the Grand Duchess and Stanisław Ponyatovsky. The article also provides updated information about Williams' personality, his activities before his arrival to St. Petersburg, the circumstances of his departure from Russia, as well as analysis of the results of his mission.