Dominic Cummings has accused Boris Johnson of "hypocrisy" for criticising MPs with second jobs as he claimed the PM wanted to devote time to writing his book on Shakespeare.
The ex-No10 aide, who has become an arch critic of his old boss, claimed Mr Johnson became bored of leading the country within a month of winning the 2019 election.
In a lengthy blogpost, Mr Cummings claimed that the Prime Minister asked him whether he could devote time working on his book to fund his "very expensive" divorce in January 2020.
Mr Johnson and his ex-wife Marina Wheeler, who split in 2018, agreed their divorce last year and he went on to marry Carrie Symonds at Westminster Cathedral in May.
No10 said Mr Cummings report of the conversation was untrue.
It comes after Mr Johnson insisted that MPs must put their parliamentary and constituency responsibilities first amid a row over second jobs.
The PM was confronted over sleaze allegations at a press conference at the COP26 summit this week, where he had to declare to world leaders that the UK was not "corrupt".
"If that system is going to continue today, then it is crucial that MPs follow the rules," he said.
"And the rules say two crucial things: you must put your job as an MP first and you must devote yourself primarily and above all to your constituents and the people who send you to Westminster, to Parliament."
Mr Cummings branded his comments as "hypocrisy" in his Substack newsletter and described a conversation in mid-January last year, where the PM called him into his study.
He stated the conversation went:
PM: "Dom, I want to run something by you. Do you think it’s OK if I spend a lot of time writing my Shakespeare book?"
DC: "What do you mean?"
PM: "This fucking divorce, very expensive. And this job. It’s like getting up every morning pulling a 747 down the runway. [Pause] I love writing, I love it, I want to write my Shakespeare book."
DC: "I think people expect you to be doing the PM’s job, I wouldn’t talk to people about this if I were you…"
Mr Cummings added: "You get the idea. Within a month of the election he was bored with the PM job and wanted to get back to what he loves while shaking down the publishers for some extra cash."
Downing Street has previously denied reports that the PM missed several COBRA meetings at the beginning of the pandemic to work on the "Riddle of Genius", his biography of Shakespeare.
