IT was always going to happen sooner or later. And always end in a Downing St bust-up.

The abrupt departure this evening of Dominic Cummings from a dysfunctional No 10 was done in his characteristically no-nonsense style.

But the chief aide's exit, coming as it does as the country prepares for the end of the transition period and the breaking of the new Brexit dawn, seems like a fitting time to draw a line in the sand and move on.

The equally swift departure of fellow Vote Leave campaigner Lee Cain, the Director of Communications, who also appears to have left No 10 for good this evening, means the ultra-Brexiteers’ grip on power and their abrasive, confrontational approach to government is finally at an end.

Of course, Messrs Cummings and Cain were always onto a loser when they made Carrie Symonds, their boss’s fiancée, an enemy. The future Mrs Johnson, herself a former Tory press chief, would have been airing her dislike of the Vote Leavers on a regular basis to the Prime Minister when the door closed for the night.

Allegra Stratton, the former broadcast journalist, who will front Downing St’s daily televised briefings in the New Year, was also opposed to the promotion of Mr Cain as No 10’s Chief of Staff, a role he was offered and then, apparently, unoffered by Mr Johnson.

Ms Stratton wants a more media-friendly approach to the Fourth Estate, which chimes with the less abrasive and more inclusive attitude Whitehall insiders have been calling for.

This week the penny finally dropped for Mr Johnson; that the plethora of policy U-turns, the dysfunction in Downing St and the disharmony generated across the Conservative parliamentary party towards the No 10 regime were making a swift end to his premiership more likely next year despite his 80-strong Commons majority.

Tory MPs, who have felt cut off from Downing St by Messrs Cummings and Cain - aides who were never of the Conservative Party - have urged their leader to use the opportunity to “reset” his administration for the challenges of 2021.

One of which, of course, will be: what to do about Scotland?

One Downing St source suggested Mr Johnson needed to adopt a “softer and less dogmatic” approach to Scotland, making the positive case for the Union. “We’ve got to make it more than just about saying ‘no’ to another referendum,” he declared.

But, of course, the PM will keep saying no to indyref2. He is, after all, the Minister for the Union and does not want to become the Minister who lost the Union.

Plans are afoot to abandon what one senior Tory figure complained of as “muscular Unionism”.

The Dunlop Report on bolstering the Union and the intergovernmental review on bolstering relations between London and Edinburgh are due out soon and will include recommendations to increase ministerial meetings and enhance communications.

Yet is Nicola Sturgeon in a mood to have a more cuddly relationship with Mr Johnson in the face of Brexit, deal or no deal, and the ongoing almighty row over the Internal Market Bill?

With the 14th consecutive poll showing a lead for the pro-independence cause, the prospect of a majority at Holyrood grows.

Momentum is very important in politics and the First Minister is not going to sacrifice that to have a nice cup of tea and a ginger biscuit with Mr Johnson.