At the start of 2020, Mr Cummings said in a blog post that he wanted to become “within a year largely redundant”.
Here’s all you need to know about Mr Cummings, his role and when he might leave Downing Street.
Who is Dominic Cummings?
Mr Cummings rose to notoriety in politics, first as an adviser to Michael Gove and then as campaign director at the official Brexit group Vote Leave.
He was portrayed by Benedict Cumberbatch in a Channel 4 drama about the Brexit campaign, which played up his role in covering a red bus with the disputed £350 million a week figure, arguing the cash could be used to fund the NHS.
Mr Cummings, a hate figure for many pro-EU politicians, said the £350 million/NHS argument was “necessary to win” the campaign.
Mr Johnson appointed Mr Cummings to his top team as senior adviser at Number 10 when he became Prime Minister in the summer of 2019.
The appointment of the abrasive former campaign director was controversial, given he was found to be in contempt of Parliament earlier in the year for refusing to give evidence to MPs investigating misinformation.
Mr Cummings has built a reputation as someone who does not play by the rules of conventional politics.
He was once called a “career psychopath” by former prime minister David Cameron, according to widely reported remarks.
But Mr Cummings is no stranger to an insult either, describing David Davis, then the Brexit secretary, as “thick as mince, lazy as a toad and vain as Narcissus” in July 2017.
The December 2019 election victory gave Mr Johnson the political capital he needed to take bold decisions – and Mr Cummings soon set to work on his goal of reshaping Whitehall, issuing a recruitment call for data scientists, economists and “weirdos and misfits with odd skills” to shake up the Civil Service.
In April, it was revealed Mr Cummings has also been present at meetings co-ordinating the response to the coronavirus pandemic as part of the Scientific Advisory Group for Emergencies (Sage).
Dominic Cummings ‘set to leave Downing Street by Christmas’
This raised concerns over a lack of breadth in expertise of the meetings and political interference in science-based advice.
The Lancet published a paper by University College London in August looking at the so-called Cummings effect, finding a “clear decrease in confidence starting on 22 May (when the story of his trip to broke) and continuing to fall quickly in the days following”.
Steve Baker, a former chairman of the Leave-backing European Research Group, described him as a “dominant figure who regards accountability with contempt”.
Writing for The Critic Magazine, he said: “To work for Dom – to obey – is to be respected, to be part of a brilliant, driven team. Dominic cultivates heartfelt and ferocious loyalty, as Vote Leave’s board found when they rightly tried to sack him for regarding accountability with disdain.
“And that, right there, is why I have always opposed Dominic being in Number 10.”
Why is he planning to resign?
Mr Cummings told the BBC that “rumours of me threatening to resign are invented” after speculation that he would follow Mr Cain in leaving Number 10.
He told the BBC’s political editor that he had indicated his plans to leave Downing Street nearly a year ago when he wrote in a January blog that he hoped to make himself “largely redundant” by the end of 2020.
The BBC also quoted a Downing Street source as saying Mr Cummings would be “out of Government” by Christmas.
Mr Cummings and Mr Cain are close political allies, having worked together since the Brexit campaign. Mr Cummings was said to be unhappy with the way his friend had been treated.
The Telegraph reported an “associate” of Mr Cain as saying the communication chief’s departure was the “beginning of the end for Dom”.
“Lee is the person who has been covering Dom’s flank 24 hours a day and he will soon be gone,” the source told the paper.
Meanwhile, Transport Secretary Grant Shapps said that Mr Cummings “will be missed” when he leaves No 10 but “advisers do come and go”.
Asked why MrJohnson’s chief adviser will be departing, Mr Shapps told Sky News: “As he wrote right at the beginning of the year in his own words, he planned to make himself largely redundant this year with the big thing that he worked on, of course, which was Brexit, coming to an end at the end of the transition period, which is December 31.
“Of course, the other big thing is helping to ensure we have the roll-out mass testing to defeat this virus. Both these things are on the near-term horizon now.
“He will be missed but then again we’re moving into a different phase and Brexit will be, we’ve already left Europe, but the transition period will be over and things move on and advisers do come and go.”
Conservative MPs have urged Mr Johnson to use events to reshape the team inside Downing Street and reconnect with the parliamentary party, some of whom feel he has been “lost” to advisers over the past year.
Additional reporting by PA Media.