Boris Johnson admitted privately that him becoming prime minister would be a “ludicrous” idea three years before he took the top job, his former aide Dominic Cummings has claimed.

In another blogpost detailing the political machinations that led up to and followed Brexit, the former No 10 adviser revealed that Johnson had admitted immediately after the referendum in 2016 that he was “unfit” to lead the country.

Cummings said Johnson had pulled him to one side in Vote Leave headquarters the morning that David Cameron resigned and the pair had discussed what should happen next.

“Obviously it’s ludicrous me being PM – but no more ludicrous than Dave or George, don’t you think?” Johnson is said to have told Cummings.

Defending why he had helped Johnson into Downing Street himself, Cummings said it was a means to “solve the constitutional crisis” caused by the Brexit stalemate – and had he chosen not to help the Conservatives, there might have been another referendum.

Cummings said he had taken the job in No 10 to “improve science, defence, Whitehall and more” – topics he had long ruminated on after his work in the civil service as a special adviser to Michael Gove.

The threat of Brexit remaining uncompleted, with Theresa May hamstrung by no parliamentary majority, could have meant another referendum, which would have left MPs unable to campaign safely outside London “without armed guards”, Cummings recalled he thought at the time.

So if the UK could finally leave the EU, “all sorts of good things” would happen, he believed.

“The problems of Boris as PM can be partly mitigated by us,” Cummings explained, “given we understand Whitehall much better than him and understand effective political action much better than him and the Conservative party. Our team will handle rough seas much better than the others.”

Cummings admitted it sounded “arrogant” but said he had been proved right on the Brexit argument, and he had believed that Johnson’s “very bad” features could be “turned to advantage” – including the prime minister’s “desire to enjoy himself rather than work hard”.

He wrote of Johnson: “Precisely because he doesn’t know what he’s doing, we may be able to get him to agree things ‘the system’ will think are ‘extreme’ but we think are necessary – like reorienting the whole state machine away from Brussels towards science and technology …

“If we win the election then he tries to move us out of No 10, we can try to move him out of No 10 – two can play at that game – and we can use reshuffles to move some much more able people into position.”

Cummings painted a picture of Johnson as a complicated man, who is “happy to hide behind the mask of a clown, mostly unbothered by ridicule, while calculations remain largely hidden (including from parts of his own mind)” but also a blatant, natural and repeated liar.

The prime minister “rewrites reality in his mind afresh according to the moment’s desire” and “there is no real distinction possible with him” between truth and lies, Cummings said.

Cummings said that when Johnson won the keys to Downing Street in July 2019, the prime minister urged him to join him as an adviser and convinced him by saying he could assemble his old Vote Leave team, saying: “All I care about is winning.”

A text Cummings recounted from Johnson read: “I’m good at motivating people, I can tell the public a story and get them behind me, but I’m not good at organising, I’m not good at all the details and dealing with the machine.”

In return, Cummings said his demand was that all special advisers would report to him and that he would be granted permission to oversee a “fundamental re-engineering of Britain’s priorities, policies, how it works”.

Cummings, who often avoided scrutiny during his time in government, has set about trying to trash Johnson’s reputation since leaving Downing Street last winter.