We saw today a chastened and humiliated Prime Minister. Boris Johnson is generally strong enough, at these encounters, to dictate the terms of debate, by indicating that he finds his opponents ridiculous.

Today, he knew it would be fatal to give any impression he was trying to laugh off the video clip which emerged last night.

Instead he set out to show that “I understand and share the anger up and down the country at seeing Number Ten’s staff seeming to make light of lockdown measures”.

Those words came in his opening statement: he would eat the gargantuan slice of humble pie which had appeared on his plate before Sir Keir Starmer could instruct him to swallow it down.

The House learned that the video clip was as repugnant to the PM as to anyone else: “Mr Speaker, I apologise unreservedly for the offence that it has caused up and down the country and I apologise for the impression that it gives.”

That is not, of course, an admission of guilt. Johnson went on to say he had been “repeatedly assured” there had been no Christmas party and no Covid rules were broken.

But he has asked the Cabinet Secretary “to establish all the facts and to report back as soon as possible”, and if the rules were broken “there will be disciplinary action for all those involved”.

Here was a major concession of power to the official machine, even if the implication was that the disciplinary action will fall on officials, not on politicians.

Sir Keir Starmer went into prosecutorial mode. Last week he had asked the Prime Minister whether there was “a Christmas party in Downing Street for dozens of people” on 18th December last year.

The Prime Minister, and the Government, had spent the week telling the public there was no such party: “Millions of people will now think the Prime Minister was taking them for fools – that they were lied to. They’re right, aren’t they?”

How Johnson would enjoy, on a normal Wednesday, making fun of his pious, prosy opponent. But today he had to say that he too was “sickened” by and “furious” about the video, and to promise that the “requisite” disciplinary action would be taken.

The Prosecutor demanded: “Will the Prime Minister support the police and support the CPS by handing over everything the Government knows about parties in Downing Street to the Metropolitan Police?”

The Accused made another concession: “Of course we will do that.”

Towards the end of the exchanges, he said Sir Keir had muddied the waters and was playing politics, but this counter-attack was launched without any of Johnson’s usual brio.

He knew that to sound frivolous would be disastrous, when so many members of the public agree with Sir Keir that this is no laughing matter.

Ian Blackford, for the SNP, called at length for the PM to resign, and Johnson replied, with brevity, that Blackford too was playing politics.

A more pointed and painful jab came from his own backbenches. William Wragg (Hazel Grove) referred to media reports that Covid Plan B was about to be implemented, and said “very few will be convinced by this diversionary tactic”.

The PM replied that “no decisions will be taken without consulting the Cabinet”.

So perhaps, like the Cabinet Secretary, the Cabinet will acquire greater power to restrain the PM.

Johnson can never have endured a more gruelling PMQs. In adversity, he was forced to strike a note of high moral seriousness, and penitence, which in prosperous days he would regard as an intolerable concession to his critics.