The publication is the first complete translation into Russian of “On the Fall of Britain” - a unique monument of Latin literature of the 6th century, a kind of open letter from St. Gilda to her compatriots and containing a story about the fall of Roman Britain and the Anglo-Saxon conquest, coming from a Briton author, who could communicate with contemporaries of these events. In addition, the chapters devoted to the denunciation of British kings and clerics provide valuable information about the British statehood and the church in the 6th century. The translation is provided with a Latin text and detailed comments. The preface outlines the handwritten history of “On the Death of Britain,” the history of the study of the monument, textual problems, and various points of view on controversial passages in the text. The author of the preface offers new hypotheses and interpretations and examines in detail the available historical data about the extraordinary personality of Gilda. The appendix to the publication contains a complete translation of two lives of Gilda, surviving fragments of his messages, as well as translations (from Latin, Irish and Old English) of biographical information about him in other medieval sources.