The written letter, obtained by The Mail newspaper from "angered insiders," revealed May’s resolve to end the "spate of corrosive leaks" even if they do not pose a security risk.
"Anyone found to have leaked sensitive information will be dismissed, even where there is no compromise of national security," the letter, attributed to Jeremy Heywood, read.
The cabinet secretary said a special security team, believed by the outlet to be MI5 domestic security officers, will be in charge of inquiries into any future leaks, including interrogation and seizure of emails and cellphones.
"The Prime Minister has directed that we urgently tighten security processes and improve our response to leaks … The Government Security team will be in touch shortly to put tighter processes in place," the message read.
The note, dated November 28, sought to end unauthorized briefings on Brexit and other sensitive issues that have recently exposed divisions within the government, composed of former June 23 referendum rivals.