Mr Johnson’s former top adviser denied he was responsible for the leak of private texts in which he promised to “fix” a tax issue for the entrepreneur Sir James Dyson.
In a lengthy blog posting he also claimed the Prime Minister had tried to stop an inquiry into the leak last year of plans for a second lockdown because it implicated a friend of his fiancee, Carrie Symonds.
He said he had also warned Mr Johnson against plans to have donors secretly pay for refurbishment of his Downing Street flat, saying they were “unethical, foolish (and) possibly illegal”.
“It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” he said.
His attack follows briefings to a number of newspapers, which said Mr Johnson believed Mr Cummings was the source of the leaks about the lockdown and his texts to Sir James as well as stories about the flat refurbishment.
It follows his dramatic departure from No 10 last year amid the fallout from a bitter power struggle with Ms Symonds.
Mr Cummings’ onslaught came as Downing Street outlined details of Mr Johnson’s communications with Sir James at the start of the pandemic.
The Government also stated formally for the first time that the Prime Minister met the cost of the Downing Street flat refurbishment out of his own pocket.
In his blog post, Mr Cummings accused the new No 10 director of communications Jack Doyle of making false accusations about him “at the PM’s request”.
He said that while Mr Johnson had forwarded him some of his WhatsApp exchanges with Sir James, they did not include those featured in the leak to the BBC.
He said that he was happy to meet Cabinet Secretary Simon Case to discuss what had happened and for his phone to be searched.
He also denied that he was the so-called “chatty rat” responsible for the leak last October which was seen to have “bounced” the Government into announcing a second lockdown.
He said Mr Case had told both him and Mr Johnson that “all the evidence” pointed to Henry Newman, a special adviser and a close friend of Ms Symonds.
The Prime Minister had said this would cause him “very serious problems” with his fiancee if Mr Newman had to be fired and asked whether the leak inquiry could be stopped.
Mr Cummings said that he had told him that was “mad” and “totally unethical” and that he could not cancel an inquiry into a leak which had affected millions of people “just because it might implicate his girlfriend’s friends”. - PA