Ministers have launched an investigation after a letter by Sir Jeremy Heywood warning against the leaking of government secrets was itself leaked to a newspaper.
Theresa May's official spokeswoman said "appropriate action" will be taken against anyone found to have been responsible for handing Sir Jeremy Heywood's internal memo to the Mail on Sunday.
In his letter to civil servants, the cabinet secretary said the prime minister wanted to see an end to a “spate of corrosive leaks” in recent months.
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“Anyone found to have leaked sensitive information will be dismissed, even where there is no compromise of national security,” he wrote.
Heywood added: "The prime minister has directed that we urgently tighten security processes and improve our response to leaks. She has instructed that we begin this work immediately and expects to see rapid and visible improvement.
"Ministers, permanent secretaries and senior officials set the tone in an organisation and no amount of process will make up for an environment where leaks are accepted. If leaders think they are the necessary cost of open ways of working they are mistaken."
But embarrassingly for May, the letter itself was then leaked to the media. Asked this morning if the Cabinet Office had launched a leak inquiry, her spokeswoman said: "The usual process for leaks is being followed."
Leak inquiries are usually handled by the Propriety and Ethics team, based in the Cabinet Office and headed up by senior official Sue Gray.
May's spokeswoman on Monday added: "We take this issue extremely seriously. That has been clear with the action that has been set out will be taken in follow up.
"The civil service code sets out very clearly the role of civil servants and how they are expected to behave with regard to official information.
"Action is being taken to investigate and follow up on that. I'm not going to get into speculation about who was behind the leaks and the action that will be taken.
"It's very clear there will be investigations and appropriate action taken if perpetrators are identified."
The fresh inquiry came as the FDA union, which represents senior officials, said any beefed-up enforcement action for civil servants should also apply to ministers who leak information.
“Ask any journalist worth their salt and they'll tell you that the vast majority of leaks emanate from politicians," said FDA general secretary 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.”