Downing Street is ordering an inquiry into a leaked document, which itself exposed a wider crackdown to stamp out government leaks.

Theresa May’s spokesperson confirmed action would be taken against “perpetrators” found to have passed the internal civil service memo to a Sunday paper.

After being asked about the leak, the spokesperson said the Prime Minister took the issue “extremely seriously”, adding: “The civil service codes set out very clearly the role of civil servants, how civil servants are expected to behave with regard to official information.

“Action is being taken to investigate and follow up on that. There will be an investigation and appropriate action will be taken if perpetrators are identified.”

In a letter to civil servants, Cabinet Secretary Sir Jeremy Heywood said the Prime Minister wanted to see an end to a “spate of corrosive leaks”, only for it then to be embarrassingly leaked to The Mail on Sunday as well.

Sir Jeremy wrote that anyone found to have leaked sensitive information would be dismissed regardless of whether the leak compromised national security or not.

He went on: “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.