The announcement that Sir Mark Sedwill, the Cabinet Secretary and National Security Advisor, has quit can be seen as the most high profile indicator that a revolution will be sweeping through Whitehall over the next four years. Sir Mark will have to be replaced, under Civil Service Commission rules, by another permanent or former permanent secretary – but the new National Security Advisor, David Frost, is currently leading the Brexit negotiations with the EU and is a darling of the Vote Leave movement. Last week, Prime Minister Boris Johnson's controversial chief advisor Dominic Cummings was reported to have warned that a “hard rain” would soon fall on the civil service.

Moreover, during a lecture at the weekend, Minister for the Cabinet Office Michael Gove laid the intellectual groundwork for the shake-up, in which he set out what he regards as the main faults in the Whitehall machine.

Mr Gove, Mr Johnson and Mr Cummings appear to be showing a united front, driven forward by the belief the country needs deep reform to succeed in a post-Brexit world.

However, unearthed reports suggest that the Chancellor of the Duchy of Lancaster and the Prime Minister have not always been the best of friends.

On the contrary, their heated rivalry has simmered and boiled for more than six years.

According to a throwback report by The Spectator, in 2014, former Prime Minister David Cameron told Mr Gove to “sink his teeth into Boris’s ankles” in a bid to halt the former Mayor of London’s chances of becoming a future Tory leader.

A minister told the magazine that the former Education Secretary had become Mr Cameron’s “licensed bear-baiter”, given free rein to “wind up” Mr Johnson.

As Tory infighting reportedly broke out, Mr Johnson's aides were accused of launching a whispering campaign to undermine Mr Gove in retaliation.

Tensions were brought into the open when former Chancellor of the Exchequer George Osborne urged Mr Johnson to stand as an MP.

Mr Johnson’s friends apparently saw the request as an underhand attempt to bind the former Mayor’s fate to Mr Cameron and therefore reduce his chances of becoming leader.

Relations deteriorated further when Mr Gove announced at a dinner with Rupert Murdoch that he would have backed George Osborne in a future leadership contest over Mr Johnson.

According to the Spectator, there was a growing anti-Gove campaign within the ranks of the Tory party, supported by Mr Johnson’s friends.

The report claims the anti-Gove group was made up of Tory ministers and MPs resentful of his closeness and influence over Mr Cameron.

Other Conservatives were said to be irritated by Mr Gove’s tendency to interrogate other ministers in meetings.

Neither side disputed that there were real differences between Mr Gove and Mr Johnson on education.

However, one friend told the magazine: “There have been some genuine tensions over education.

"But it doesn’t account for all this madness.”

The Mayor demanded more power to intervene in failing schools in the capital – a request that was later rebuffed by Mr Gove.

The tensions between the former Education Secretary and Mr Johnson did not end there, though.