Dominic Cummings has launched an unprecedented and extraordinary attack on Boris Johnson, alleging that the prime minister tried to quash a leak inquiry as it implicated an ally, and hatched a “possibly illegal” plan for donors to pay to renovate his flat.

The outburst by Cummings, a day after anonymous No 10 sources claimed that he had leaked private text messages between Johnson and the billionaire James Dyson, prompted Labour to accuse the government of “fighting each other like rats in a sack”.

Cummings used a lengthy post on his personal blog to deny any leaking. Instead, he accused Johnson and his team of a series of wrongdoings. He said the prime minister had behaved in a way he considered “mad and totally unethical”, and warned that he would happily give evidence under oath to an inquiry.

“It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” he wrote.