When friends go their separate ways after a row, it’s sometimes because their quarrel ran deeper: the split was coming.  Here lies the most convincing explanation of Munira Mirza’s parting from Boris Johnson.

Others don’t stack up.  Was she somehow compromised by “partygate”?  She has not been reported as present at anything like a party, and her former Policy Unit colleagues have been absent from the story (at least so far).

Was she jumping before she was pushed?  It’s unlikely since, as a non-party goer, she wasn’t compromised.  The Prime Minister will have wanted some continuity amidst the departures.

Or is the most simple explanation the real one: namely, that she objected to the way in which the Prime Minister linked Keir Starmer’s name to Jimmy Savile, and walked?

I’ve no special insight into Mirza’s thinking, but believe that the answer runs deeper.  I suspect that a growing number of his closest supporters are giving up on him.

And the former head of the Policy Unit is – or I should now write “was” – not just a close supporter but a long-time one, having first worked with Johnson as his cultural adviser when he was Mayor of London.

She is as close to being a friend of his as her role as a staffer allowed. “I don’t think I have ever met anyone so efficient, and with such a horror of wasting taxpayers’ money, the Prime Minister has said of her.

“She hates cant; she hates frippery; she hates political correctness. She has, all told, the most powerful nonsense-detector I have ever seen.”  Now the detector has had enough of his nonsense (as she sees it).

Consider the roll-call of senior members of the Prime Minister’s staff who left Number Ten before she did. ord Udny-Lister, Chief Strategic Adviser, then Chief of Staff…

…Dominic Cummings, Chief Adviser to the Prime Minister. Lee Cain, Director of Communications. Allegra Stratton, Press Secretary. James Slack, Director of Communications…

…Nikki da Costa, Director of Legislative Affairs.  Oliver Lewis, Head of the Union Unit. To which we must in Mirza’s wake: Dan Rosenfield, Chief of Staff.  Jack Doyle, Press Secretary. Martin Reynolds, Private Secretary…

…And, talking of senior civil servants, Mark Sedwill, Cabinet Secretary.  And talking of the Cabinet: David Frost has quit.

Let’s leave aside for the moment whether Mirza was right to go – though I point out in passing that the Sewell Report, which she helped to bring to the light of day, awaits a Government response the best part of a year on.

At the heart of Sewell was the view that ethnic minorities are not always disadvantaged compared to the majority – and that not all of them are the same as each other.

I described it at the time as “a report so commonsensical but consensus-challenging that we’re surprised it was allowed to happen”.

Johnson has failed to back Sewell up and, if there is a culture war, it’s one in which he’s unwillingly enlisted. Mirza is one of the few rightists who knows her way round the politics of culture, and her departure leaves a gap.

Whether you think the Prime Minister should or shouldn’t be so engaged, the facts of the bigger case are these.  An extraordinarily large number of senior staff have left his Number Ten in little more than two years.

Some have jumped, some have been pushed, and some jumped while being pushed.  Others will now be pushed or jump or both as the police continue their enquiries and the Gray Report hangs Damocles-like over Downing Street.

Furthermore, the clearout that Johnson has promised Conservative backbenchers will sweep away yet more SpAds and staff.

Yesterday evening’s rushed-out news of Rosenfield’s, Reynold’s and Doyle’s departure was clearly intended both to distract from Mirza’s resignation and get bad news out of the way at once.

However, it is only the start.  Or if it isn’t, Johnson will soon have the 1922 Committee’s Executive, plus a mass of other Tory MPs, knocking at his door and demanding that he honour his commitments.

So a mass of staff are heading for the exit.  But will a mass of the brightest and the best queue up to enter?  Many will ponder the roll-call of departures above, and look no further.

It is significant that Johnson has hurried to fill the gap left by Mirza’s departure with an MP: one of his two Parliamentary Private Secretaries – Andrew Griffith.

The appointment may be a signal to Conservative MPs that from now on the Prime Minister will rely on them rather than advisers.  Many will greet such a message with a raised eyebrow (perhaps with two).

And with reason.  Earlier this week, it was briefed that Lynton Crosby will return to Johnson’s side to save the day.  He is currently denying everything from Australia.

You may dismiss Mirza’s departure, plus all these others, as Westminster processology of no interest or relevance to ordinary people.

And add that dignifying them with an editorial is using up space better concentrated on rising taxes, small boats, Net Zero, a deregulation mouse where there should be a lion, waste, immigration and wokery.

Not to mention the prospect of the biggest squeeze on living standards in a generation.  To which I say that none of these issues will be sorted, or even addressed, in a Downing Street that is comprehensively dysfunctional.

The fact is that the wheels of government are grinding very slowly when they work at all.  Michael Gove can’t be permanently on hand to take the initiative and change the subject, as he did earlier this week over Levelling Up.

The Prime Minister’s supporters argue that he is the victim of a Remain, woke, Labour, deep state, BBC and leftist plot to oust a proven Conservative election winner with an unaltered electoral mandate.

Both our own surveys and opinion polls elsewhere suggest that opinion among Party members is divided.  At any rate, the prospect ahead for both them and Tory MPs is challenging.

Sue Gray’s report may leak.  If the police don’t they will be flouting one of the great traditions of the constabulary.  They may interview the Prime Minister himself.

Meanwhile, some of those who have left Downing Street will tell their side of the story.  It will not always reflect well on Johnson himself.

Perhaps he will break free, as so often before.  But as matters stand it looks as though he won’t be able to get this monkey off his back.

It’s impossible to know whether or not Mirza’s departure will help provoke a leadership challenge.  My sense is that some of the Prime Minister’s internal opponents don’t want one. Yet.

That’s because they think he is more likely to win a ballot now than, say, after May’s local elections or police action or Gray’s report is published.

But neither the Party nor the country should have rely on backbenchers to shape the future.  Johnson’s Ministers have a special responsibility as Ministers in the besieged Government for which they work to get a grip.

In the ideal world that doesn’t exist, Dominic Raab, Rishi Sunak, Liz Truss and Michael Gove would band together and go to the Prime Minister with an ultimatum.

Which would be: for whatever reason, this Government isn’t working – for all its successs, principally the vaccine rollout.

From now on, it should be run as a collective enterprise with the Cabinet top team making collective and – dare I add – Conservative decisions.

A recipe for chaos?  Perhaps.  But what’s the alternative?  If the Cabinet won’t act, I begin to ask what the point of it is – all the way to the top.