On 23 July 1914, the Austro-Hungarian Minister of Foreign Affairs Leopold Count Berchtold (pictured) sent a coded private telegram to the Austro-Hungarian Ambassador to the United Kingdom Albert Count Mensdorff. In his telegram, Berchtold issues instructions on the distribution of the content of the Austro-Hungarian ultimatum to Serbia.
Count Berchtold to Count Mensdorff. Vienna, 23 July 1914. Vienna, July 23, 1914.
Received your Excellency’s telegram of 22 July.
Your Excellency may in today’s conversation with [British Foreign Secretary] Sir Edward Grey announce the official presentation of the circular note for tomorrow morning [i.e. the Austro-Hungarian ultimatum to Serbia] and, in strict privacy communication, the contents of the note, asking him to keep what you tell him, strictly private.
Source: 1919 Austro-Hungarian Red Book, with minor edits.