Cabinet secretary Sir Mark Sedwill has written to all departmental permanent secretaries to emphasise that unauthorised briefings to the media "will not be tolerated" amid a row over claims about Jeremy Corbyn's health.
The letter – dated 1 July and revealed by the Guido Fawkes website – comes after The Times quoted unnamed civil servants claiming Corbyn is too "frail" to become prime minister and suggesting that he has health problems.
Those claims have been dismissed as “a farrago of nonsense” by the Labour leader.
The Cabinet Office has launched an investigation into whether the comments amount to a breach of the civil service code – although Corbyn has demanded a fully independent probe in order to ensure "trust" in the process.
The letter from Sir Mark says the civil service has "loyally supported" Theresa May over the past three years, and will need "to do the same for a government formed under a new prime minister, as we also would do were there to be a change of the party in government".
He adds: "In particular, I want to be very clear that unauthorised briefing to the media about politicians or policy will not be tolerated. It is incredibly damaging for the overall integrity of the civil service and can undermine the confidence that ministers – and ministers in future governments have in the civil service. It is a breach of the Civil Service Code and disciplinary action will be taken against anyone who has been found to be in breach of this provision."
Sedwill, who is also the head of the civil service, urged senior officials to "discuss the values" of the organisation with staff "and encourage them to ask their senior civil service colleagues to be thinking about them as part of planning for forthcoming changes".
The Cabinet Office told CSW that it did not comment on leaked documents, adding that impartiality is one of the fundamental values of the civil service and underpins its ability to effectively serve the government of the day, and that the comments made by civil servants in Saturday's Times are being investigated as a potential breach of the civil service code.