On the one hand, it is hard to argue that being investigated by the police for an alleged breach of the law is a good look for a serving prime minister. 

On the other, such is the weight of expectation on Ms Gray’s findings that kicking her report into the long grass may feel like something of a blessing. 

David Davis, the former leadership contender, is among a number of MPs who have taken Mr Johnson at his word and vowed to wait until the Cabinet Office report is published before deciding whether to submit a letter to Sir Graham Brady, chairman of the 1922 Committee of Conservative backbenchers, calling for a vote of no confidence in Mr Johnson’s leadership. 

It takes 15 per cent of the parliamentary party – amounting to 54 MPs – to trigger a contest.

So delaying that moment, potentially the most dangerous of Mr Johnson’s entire premiership, gives him breathing space. 

Who knows what might happen in the interim: Russia could invade Ukraine; a new and yet more deadly coronavirus variant might emerge; the energy crisis may focus minds on something more crunchy politically than birthday cakes and wine and cheese. All three of these developments would be devastating for the globe; rather helpful to the Prime Minister.

But while Mr Johnson may have felt a slight easing of pressure in the immediate term from Dame Cressida’s bombshell announcement, the reprieve is by necessity temporary. 

One option said to be under consideration is to split the Gray inquiry into two, allowing her to publish her findings in relation to any gatherings not subject to police investigation within the next week or so, with all the fallout that is likely to follow.

Even if this option is not pursued, the timings of the eventual outcome of the policy inquiry and resumed Cabinet Office probe are likely to be crucial to Mr Johnson’s prospects for survival.

Met sources have suggested the criminal investigation is likely to take “weeks not days.” That takes it well into spring – perilously close to the May local elections.

Should the Prime Minister be found to have broken the law himself, or to have overseen a regime in Number 10 which encouraged or facilitated the breaking of rules it set for others, it would be hard to see how he remains in office.

Even if the police and Ms Gray’s findings fall short of that benchmark, a severely critical report could still colour the May elections.  

That is likely to focus Tory MPs’ minds. It is often said that Conservatives don’t like a loser – if Mr Johnson leads them to heavy defeat in May, they are unlikely to stick with him for long after that.

Rosa Prince is Editor of The House magazine