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.
This raised concerns over a lack of breadth in expertise of the meetings and political interference in science-based advice.
Mr Cummings had previously been observed failing to follow the two-metre social distancing rules as he walked along Downing Street flanked by fellow aide Cleo Watson on April 14.