Former Downing Street aide Dominic Cummings said he was looking to oust Boris Johnson as Prime Minister only weeks after helping him secure an 80-seat majority.
Mr Cummings, who left No 10 in the autumn after a power struggle, accused Mr Johnson of not having a plan and said he “doesn’t know how to be Prime Minister”.
In an interview with the BBC, the Vote Leave mastermind said he assisted the Conservative Party poll victory in December 2019 in order to settle the Brexit debate rather than due to any firm belief in his leadership.
Mr Cummings also laid bare the extent of the fractious relationship between former Vote Leave officials and Mr Johnson’s now-wife, Carrie Johnson only weeks after the landslide win.
“Before even mid-January we were having meetings in Number 10 saying it’s clear that Carrie (Johnson) wants rid of all of us,” said the former de facto chief of staff.
“At that point we were already saying by the summer either we’ll all have gone from here or we’ll be in the process of trying to get rid of him and get someone else in as Prime Minister.”
Mr Cummings claimed that in 2019, ahead of the election, Mrs Johnson was happy to have Vote Leave officials working in Downing Street, but this later changed.
He said: “As soon as the election was won her view was, ‘why should it be Dominic and the Vote Leave team?’ Why shouldn’t it be me that’s pulling the strings?'”
In comments due to be aired on Tuesday evening, Mr Cummings was less than complimentary about Mr Johnson’s vision for the country.
He added: “He doesn’t have a plan, he doesn’t know how to be Prime Minister and we only got him in there because we had to solve a certain problem not because he was the right person to be running the country.”
Formerly an aide to Michael Gove when he was education secretary, Mr Cummings said his relationship with Mr Johnson was starting to break down “by summer 2020”, with him and former director of communications Lee Cain departing by November.
Explaining the split, Mr Cummings said the Prime Minister was “fed up with the media portrayal of him being a kind of puppet for the Vote Leave team, it was driving him round the bend”.
He also said there were disagreements over the strategy on handling the pandemic, for improving the country and over the-now Mrs Johnson’s increasing influence over how Government was run.
“I had a plan, I was trying to get things done, he didn’t have a plan… he didn’t have an agenda,” he continued.
“You know the Prime Minister’s only agenda is buy more trains, buy more buses, have more bikes and build the world’s most stupid tunnel to Ireland – that’s it,” he said.
“Also he knew that we basically disagreed about what was happening on Covid and he knew that I was blaming him for not having acted in September, which I was.”
The pair had a “big argument”, according to Mr Cummings, after Mr Johnson’s then-girlfriend called for people to be fired or promoted “in ways that I thought were unethical and unprofessional”.
On Brexit, Mr Cummings said that anyone who was “sure about questions” like Britain’s divorce from the European Union has “got a screw loose” but argued it had so far been “a good thing”.
“I think it’s perfectly reasonable to say Brexit was a mistake…of course it’s reasonable for some people to think that,” he continued.
“I think that obviously I think Brexit was a good thing… I think that the way in which the world has worked out since 2016 vindicates the arguments that Vote Leave made in all sorts of ways. I think it’s good that Brexit happened.”
A Number 10 spokesman said: “Since the start of the pandemic, the Prime Minister has taken the necessary action to protect lives and livelihoods, guided by the best scientific advice.
“The Government he leads has delivered the fastest vaccination rollout in Europe, saved millions of jobs through the furlough scheme and prevented the NHS from being overwhelmed through three national lockdowns.
“The Government is entirely focused on emerging cautiously from the pandemic and building back better.”
The spokesman added: “Political appointments are entirely made by the Prime Minister.”