For days, the attacks had been raining down from Elon Musk and his supporters on Keir Starmer, safeguarding minister Jess Phillips and the wider government, over handling of the historical sex abuse cases. 

The consensus in Number 10, as voiced by another leader subject to Mr Musk’s ire – German Chancellor Olaf Scholz – was do “not feed the troll”.

And so, as Mr Musk posted dozens of times about sex grooming gangs in the north of England, and accused Ms Phillips of being a “rape genocide apologist” and a “wicked witch”, the government kept out of the fray.

But that all changed on Monday when the PM came out swinging, with the most impassioned remarks I can remember him making, when I asked him to comment on Mr Musk’s abuse of Ms Phillips on social media.

He said the debate on child sex exploitation was based on lies, with politicians “jumping on the bandwagon simply to get attention”, as he hit back at not just at Mr Musk but the leader of the Opposition Kemi Badenoch too.

“We have seen this playbook many times – whipping up of intimidation and of threats of violence, hoping that the media will amplify it,” he told me on a visit to Epsom Hospital.

“When the poison of the far right leads to serious threats to Jess Phillips and others, then in my book a line has been crossed.

“I enjoy the cut and thrust of politics, the robust debate that we must have. But that’s got to be based on facts and truth, not on lies. Not on those who are so desperate for attention that they’re prepared to debase themselves and their country.”

From a lawyerly prime minister that chooses his words carefully, and is often ponderous in his approach to questions, this was quite a handbrake turn.

For months, Number 10 has brushed off repeated attacks on the PM and Labour government from Mr Musk and in many respects that made sense.

The tech billionaire is in Donald Trump’s inner circle and about to have an official role in his administration.

He perhaps worries that taking on Mr Musk will anger President Trump and make an already delicate relationship even harder to navigate on critical issues such as tariffs and support for Ukraine. So why, on Monday, did he bite back?

Firstly, I’m told the PM is angered that the abuse and disinformation online has led to threats to Ms Phillips, with one man being charged with malicious communication to the MP over the weekend.

“It crosses the line into MPs’ safety,” said one Number 10 insider.

“There will be people who say ‘don’t feed the troll’ and I think Keir Starmer is of that view and wanted to avoid getting into that side of things,” they said.

But there are times when the responsibility of a PM “is to try to shape these issues”, they added.

“His view was this is a moment where he has a responsibility to say he’s happy to have a debate and stand by his record, but there are some things we can’t accept as a country – and that is misinformation and disinformation around individuals.”

Starmer is frustrated about the disinformation that Mr Musk is spreading online and believes it is “dangerous” not just to individual MPs but to UK democracy.

On Monday, he criticised those who had been defending far-right agitator Tommy Robinson, who the PM said “went to prison for nearly collapsing a grooming case”.

“These are people who are trying to get some kind of vicarious thrill from street violence that people like Tommy Robinson promote.”

Starmer defended his record as chief prosecutor, as Mr Musk accused the prime minister of being “deeply complicit in the mass rapes in exchange for votes”.

Some of the online allegations levelled at Starmer seem to refer to his time at the Crown Prosecution Service (CPS) and the failures more than a decade earlier to bring grooming gangs to justice: In 2009 a decision was taken not to prosecute alleged perpetrators in the town of Rochdale after lawyers believed the victim would not come across as credible.

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“For many, many years, too many victims have been completely let down; let down by perverse ideas about community relations or by the idea that institutions must be protected above all else. And they’ve not been listened to, and they’ve not been heard,” said Starmer on Monday.

“And when I was a chief prosecutor for five years, I tackled that head-on, because I could see what was happening, and that’s why I reopened cases that have been closed and supposedly finished. I brought the first major prosecution of an Asian grooming gang – in the particular case it was in Rochdale, but it was the first of its kind.”

He said his record was “not secret” and that he has called for “mandatory reporting of child sexual abuse” when he was at the CPS, which “the Tories did nothing about.”

Third, it is about standards in public life, with the prime minister clearly irritated with Ms Badenoch’s and others’ handling of the affair as he called out politicians for not disavowing the attacks on Ms Phillips.

“He thinks truth matters in politics and while politics is a place of skullduggery, it also needs to be a place based on fact and truth and not smears,” said one aide.

For their part, the Conservatives on Wednesday will attempt to amend the Children’s Wellbeing and Schools Bill to require a full national inquiry into grooming gangs, with Ms Badenoch saying on X that her party’s action will “do right by the victims and end the culture of cover-ups”.

There are still questions about whether there should be a wider inquiry into historical sex abuse and the “endemic” abuse Professor Alexis Jay referred to in her 2022 inquiry.

Calling for it on the back of the Musk interventions in recent days has been the first big intervention by Ms Badenoch two months into her leadership.

The PM on Monday defended his decision not to have another review, saying that the Jay Report published in 2022 was a “comprehensive review” of child exploitation which “doesn’t need more consultation or research. It just needs action”.

The home secretary took such action today by announcing in the Commons that she would strengthen the laws around child sexual exploitation through the implementation of Professor Jay’s recommendations.

But the actions are unlikely to end this row, with Ms Badenoch demanding a national inquiry over this issue as she seizes on grooming gangs and historical abuse as a dividing line with Labour on which she believes she can take on Starmer and go toe to toe with Reform.

It was not the start to the new year that the prime minister intended.

But it is true, too, that Number 10 decided that, after ducking the Musk attacks for months, this was a ground he would not cede, with the PM making a deliberate choice to kick off the new year facing down these attacks.

And it was perhaps the most authentic I have seen him for months as he answered criticisms of his past record as a prosecutor and denounced the vicious treatment of his own MP.

An embattled prime minister since entering Number 10, he’s come out in 2025 fighting.