The article deals with a specific episode from the diplomatic history of the Trieste Question - namely, the British territorial assurances, given to the government of the Kingdom of Yugoslavia in the spring of 1941, as well as the trace of these long unclarified episode in historiography and memoirs. For a long time it remained questionable whether these assurances had actually been given and if so, in what form, and especially whether Trieste was truly included, as was frequently stated mainly in the texts of Italian authors. From the critical survey of interpretative theses on the nature and territorial extent of assurances it can be seen that despite the partial clarification of some questions, controversies did not end with the declassification of the British archives in the beginning of the 1970s, but still persist in the form of mutually completely unreconciliable and contrary theses.