The Government is mounting a leak inquiry into the leaking of a letter warning civil servants not to leak, Downing Street has indicated.
The memo from Cabinet Secretary Sir Jeremy Heywood, telling senior officials the Prime Minister had demanded urgent action to tighten security in Whitehall, was obtained by The Mail on Sunday just days after being issued.
The letter warned that anyone found to have leaked sensitive information would be dismissed, even where there was no compromise of national security.
But a union representing senior civil servants said that politicians, not officials, were most likely to be responsible for leaks and that they too should be threatened with "the loss of their livelihood" if they were caught.
"Ask any journalist worth their salt and they'll tell you that the vast majority of leaks emanate from politicians," said the general secretary of the FDA union Dave Penman.
"Indications have been given that the Prime Minister will issue a parallel and similarly robust statement outlining that any politician found to have leaked will face a similar sanction. This would be both welcome and appropriate.
"Civil servants are being threatened with the loss of their livelihood if they are caught leaking. A similar sanction should apply to politicians, not simply the loss of the trappings of ministerial office."
Asked whether an inquiry would be launched into the unauthorised release of Sir Jeremy's memo, Theresa May's official spokeswoman said: "The usual process for leaks is being followed. Action is being taken to follow up and investigate that.
"There will be investigations and appropriate action taken if perpetrators are identified."
The spokeswoman said that unauthorised disclosures were taken "extremely seriously" and that the civil service code made clear how officials should behave with regard to classified information.