The Prime Minister’s former chief adviser told the BBC he held discussions about trying to force out the Prime Minister just days after the result.
In his first TV interview aired on Tuesday night, Mr Cummings also claimed he still believed in Brexit, but admitted it was “reasonable” for others to label it a mistake.
He also alleged the Prime Minister’s wife Carrie Johnson had tried to influence government appointments, saying: “Before even mid-January we were having meetings in Number 10 saying it’s clear that Carrie wants rid of all of us.
"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.
“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.”
The former adviser said there was a problem because the Prime Minister’s partner’s attitude changed.
He continued: “Carrie’s view was and is the Prime Minister doesn’t have a plan and he doesn’t know how Whitehall works, someone is going to settle the agenda. It can either be the civil service or it can be Dominic and the Vote Leave team or it can be me.
"In 2019 her view was better that it’s Dominic and the Vote Leave team than the civil service because that’s the route to winning and staying in Number 10.
"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?’
“The situation we found ourselves in is that within days we were in a situation where the Prime Minister’s girlfriend is trying to get rid of us and appoint complete clowns to certain key jobs.”
The former Vote Leave mastermind also claimed anyone sure about Brexit “has got a screw loose”.
He explained: “I think one of the reasons why we won is precisely, in Vote Leave we didn’t think that we’re definitely right and Remainers are all idiots or traitors or anything else.
"We never thought like that then and still don’t and I don’t know. I think it’s perfectly reasonable to say Brexit was a mistake and the little, the history will, will, will prove that. Of course it’s reasonable for some people to, to think that.
“But questions like is Brexit a good idea? No-one on earth knows if that’s, what the answer to that is. I think that obviously, I think Brexit was a good thing."
Mr Cummings also claimed his relationship with Mr Johnson has started to break down “by summer 2020” for a number of reasons.
Criticising Mr Johnson’s plan to build an undersea link between Scotland and Northern Ireland as the “world's most stupid tunnel”, Mr Cummings explained: “First of all he [Boris Johnson] was, he 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.
“I had a plan, I was trying to get things done. He didn’t have a plan … he didn’t have an agenda. 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.
“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.
"And then … I thought that his girlfriend was interfering with ‘erm, appointments. She wanted to have people fired and she wanted to have people promoted in ways that I thought were unethical and unprofessional. And that also led to a big argument between us.”
The Brexiteer also insisted it was not about getting revenge, having been let go by Downing Street.
He said: “The reason I’m speaking out is I want people to be thinking about these questions.
“How are we governed? How is power actually exercised in No.10? What sort of things should be more transparent? How should these power structures be opened up?”
“It doesn’t matter if it’s personal. It doesn’t matter if people are upset. All these MPs or ministers or officials or whoever.
"We need more difficult conversations in this country, more people upset.”
A Number 10 spokesperson 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.
“Political appointments are entirely made by the Prime Minister.”