When Number 10 briefed newspapers on Thursday that Dominic Cummings was the source of leaks of the Prime Minister’s text conversations with Sir James Dyson and the Crown Prince of Saudi Arabia, and pointed to Cummings as the ‘chatty rat’ who leaked news of the November lockdown, I said this looked like an exercise in mutually assured destruction.

And so it has turned out. The PM’s former chief aide – who was closer to Boris Johnson than anyone till he was forced out at the end of last year – has issued a statement that explicitly brands Johnson as ‘unethical’ and implicitly calls him a liar.

Cummings says he does not possess the relevant Dyson texts, so could not be the leaker, and that he can prove the PM knows both that he is not the ‘chatty rat’ and that the finger of suspicion was pointed at another aide, Henry Newman.

He recalls the PM saying to him:

‘If Newman is confirmed as the leaker than I will have to fire him, and this will cause me very serious problems with Carrie [the PM’s fiancée] as they’re best friends…[pause] perhaps we could get the Cabinet Secretary to stop the lead enquiry.’

Cummings says:

‘I told him that this was ‘mad’ and totally unethical, that he had ordered the inquiry himself and authorised the Cabinet Secretary to use more invasive methods than are usually applied to leak inquiries because of the seriousness of the leak. I told him that he could not possibly cancel an inquiry about a leak that affected millions of people, just because it might implicate his girlfriend’s friends.’

As I understand it, Cummings has WhatsApp messages that substantiate his claims that he was exonerated as the leaker, and that the Cabinet Secretary Simon Case believed ‘all the evidence definitely leads to Henry Newman and others in that office’.

This rebuttal from Cummings harms the PM directly, because they relate to briefings from Downing Street that were authorised by the PM and in his name.

In fact I am told that some of yesterday’s briefing of editors was by Johnson himself (I have put this to Downing Street and they have responded). So Cummings is directly impugning the PM’s honesty.

Nor is that the worst charge Cummings lays at his door, because he is equally incensed at the claim that he leaked details of how the PM tried to have the costs of refurbishing his Downing Street paid by Tory party donors. Cummings says:

‘The PM stopped speaking to me about this matter in 2020 as I told him I thought his plans to have donors secretly pay for the renovation were unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations if conducted in the way he intended. I refused to help him organise these payments’.

For the avoidance of doubt, therefore, this argument between Cummings and the PM is a full scale political crisis for the government.

It is quite difficult to see how Henry Newman can remain in his post unless he can prove he was not the source of the lockdown leak.

But more grave for the PM is that Cummings wants to put ‘some private text messages’ in the public domain, ‘because of the seriousness of the claims being made officially by No 10’.

Also he wants the whole matter of who is telling the truth examined ‘via an urgent Parliamentary inquiry into the government’s conduct of the Covid crisis which ought to take evidence from all key players under oath and have access to the documents’.

Cummings will ‘cooperate fully with any such inquiry and am happy to give evidence under oath’. He is also ‘happy for No 10 to publish every email I received and sent July 2019 to November 2020’.

On 26 May, he is scheduled to appear before MPs on the joint health and science super committee and will ‘answer questions about any of these issues… for as long as the MPs want’.

And just to give you a preview, he will confirm what I have said about Cummings in the past that 1) he was urging the PM to lock down in March earlier than the PM was prepared to do, and 2) he wanted the PM to lock down in September, but the PM refused.

The fundamental point is that long before Cummings was forced out of government in November, he was becoming disillusioned with Johnson’s stewardship of the country and his management of the Covid-19 crisis in particular. Having been attacked by the PM, he wants to set the record straight – which, to put it mildly, is not a prospect that Johnson will relish.

PS. Downing Street has just issued the following statement, which feels a bit like using a watering can to put out a blazing inferno. It reads:

‘This government is entirely focused on fighting coronavirus, delivering vaccines and building back better.[on the Downing Street flat]. At all times, the government and ministers have acted in accordance with the appropriate codes of conduct and electoral law. Cabinet Office officials have been engaged and informed throughout and official advice has been followed. All reportable donations are transparently declared and published – either by the Electoral Commission or the House of Commons registrar, in line with the requirements set out in electoral law. Gifts and benefits received in a ministerial capacity are, and will continue to be, declared in transparency returns.’

On the leaks:

‘The PM has never interfered in a government leak inquiry.’

Make of all that what you will. But it conspicuously fails to answer Cummings’s central charges.