- Coronavirus pandemic
image copyrightPA Mediaimage captionPictures of Mr Cummings leaving No 10 dominated newspaper front pages
The prime minister's former chief adviser, Dominic Cummings, is still a big name in Westminster, even though he's no longer at No 10.
Once a hugely powerful figure, he left his Downing Street role at the end of last year, following an internal power struggle.
But he has continued to make headlines, attacking politicians and officials alike for their handling of the Covid-19 pandemic.
He continued that criticism on Wednesday, when he gave explosive evidence to MPs investigating how the government has handled the crisis.
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However, it was not the first time he has raised eyebrows in No 10 since leaving his post.
After being named in media reports as the source of government leaks, he launched a scathing attack on Mr Johnson via a 1,000-word blog post in April.
As well as denying he was behind the leaks, he went on to make a series of accusations against the PM and questioned his "competence and integrity".
Despite his supposedly backroom role, Mr Cummings has achieved remarkable prominence, and indeed notoriety.
Barnard Castle trip
His trip to his parents' home in County Durham during the first national lockdown, when his wife was suffering Covid symptoms, became the stuff of social media legend, dominating the tabloid front pages and the national conversation for days.
media captionCummings drove to Barnard Castle to test vision
It also succeeded in putting the historic market town of Barnard Castle on the map.
Mr Cummings said he had visited the town, with his family, to test his ability to drive back to London, after experiencing loss of vision due to coronavirus.
This explanation – which launched a thousand memes – came during an unprecedented press conference in the Downing Street garden to explain the trip.
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He said he had travelled to Durham because he believed both he and his wife were about to be laid low with coronavirus and he wanted to guarantee his four-year-old son's safety.
"I don't regret what I did," he told journalists. "The situation I was in was exceptional circumstances. I think I behaved reasonably."
Boris Johnson stood by his adviser throughout this episode – to the consternation of some of his supporters, who feared it was undermining his attempts to hold the country together during a national crisis.
Some said the episode burned through the political capital the prime minister had generated months earlier during the 2019 general election.
media captionBoris Johnson defends his senior adviser Dominic Cummings in May 2020
But it is hard to overstate how important Mr Cummings was to the Johnson project.
The two are very different characters – Mr Johnson likes to be popular, Mr Cummings appears indifferent to such concerns – but they formed a strong bond in the white heat of the 2016 Vote Leave campaign to get Britain out of the EU, which Mr Cummings led as campaign director.
The combination of Mr Johnson, the flamboyant household-name frontman, with Mr Cummings, the ruthless, data-driven strategist, with a flair for an eye-catching slogan, proved to be unbeatable.
Mr Cummings was credited with formulating the "take back control" slogan that appears to have struck a chord with so many referendum voters, changing the course of British history.
Yet some were surprised when Mr Cummings was brought into the heart of government as Mr Johnson's chief adviser, given his past record of rubbing senior Tory politicians up the wrong way.
It proved to be a shrewd move. It was Mr Cummings who devised the high-risk strategy of pushing for the 2019 election to be fought on a "Get Brexit Done" ticket, focusing on winning seats in Labour heartlands, something no previous Tory leader had managed to do in decades.
Many of the policy ideas that have shaped the Johnson government's agenda have his fingerprints all over them.
"Levelling up" – moving power and money out of London and the South East of England – is a Cummings project, as are plans to shake-up the civil service, take on the judiciary and reform the planning system.
The team that surrounded Mr Cummings at Downing Street, some of whom are Vote Leave veterans, were fiercely loyal to him and shared a sense that they were outsiders in Whitehall, battling an entrenched "elite".
Lee Cain, the former Downing Street and Vote Leave communications chief, left No 10 just before Mr Cummings did.
image copyrightReutersimage captionMr Cummings watches the PM in action at a coronavirus briefing
There had been rumours of a rift between the Vote Leave veterans and other No 10 aides, who didn't like Mr Cummings's and Mr Cain's abrasive style.
There were tales of crackdowns on special advisers suspected of leaking to the media and angry, dismissive behaviour towards Tory MPs, civil servants and even secretaries of state.
None of this will have come as much of a surprise to veteran Cummings watchers.
'Weirdos and misfits'
Mr Cummings has been in and around the upper reaches of government and the Conservative Party for nearly two decades, and has made a career out of defying conventional wisdom and challenging the established order.
But he has never been a member of the party, and has little time for what he sees as some of the time-servers and publicity addicts that populate the Commons benches.
A longstanding Eurosceptic who cut his campaigning teeth as a director of the anti-euro Business for Sterling group, Mr Cummings's other passion is changing the way government operates.
He grabbed headlines when he posted an advert on his personal blog for "weirdos and misfits with odd skills" to work in government.
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The career civil servants and "public school bluffers" in Whitehall are singularly ill-equipped to take decisions about complex issues, he has argued at great length on his blog.
Instead, he believes, mathematicians and data scientists should be given a far bigger role – and he has made an effort to educate himself in these fields.
image copyrightAFPimage captionThe Vote Leave bus was one of its most notable campaign tactics
As the world now knows, Mr Cummings is a native of Durham, in the North East of England. His father, Robert, was an oil rig engineer and his mother, Morag, a teacher and behavioural specialist.
He went to a state primary school and was then privately educated at Durham School. He graduated from Oxford University with a first-class degree in modern history and spent some time in Russia, where he was involved with an ill-fated attempt to launch an airline, among other projects.
After a stint as campaign director for Business for Sterling, he spent eight months as chief strategy adviser to then Conservative leader Iain Duncan Smith.
He played a key role in the 2004 campaign against an elected regional assembly in his native North East.
In what turned out to be a dry run for the Brexit campaign, the North East Says No team won the referendum with a mix of eye-catching stunts – including an inflatable white elephant – and snappy slogans that tapped into the growing anti-politics mood among the public.
He is then said to have retreated to his father's farm, in County Durham, where he spent his time reading science and history books in an effort to attain a better understanding of the world.
image copyrightEPAimage captionMr Cummings facing questions from the media outside his home
He re-emerged in 2007 as a special adviser to Michael Gove, who became education secretary in 2010 and turned out to be something of a kindred spirit.
The pair would rail against what they called "the blob" – the informal alliance of senior civil servants and teachers' unions that sought, in their opinion, to frustrate their attempts at reform.
He left of his own accord to set up a free school, having alienated a number of senior people in the education ministry and the Conservative Party.
He once described former Brexit Secretary David Davis as "thick as mince" and as "lazy as a toad" and irritated David Cameron, the then prime minister, who called him a "career psychopath".
image copyrightChannel 4/House Productionsimage captionBenedict Cumberbatch was widely praised for his portrayal of Dominic Cummings in Brexit: The Uncivil War
His appointment as head of the Vote Leave campaign – dramatised in Channel 4 drama Brexit: The Uncivil War – was seen as a risk worth taking by those putting the campaign together but he left a controversial legacy.
Vote Leave was found to have broken electoral law over spending limits by the Electoral Commission and Mr Cummings was held in contempt of Parliament for failing to respond to a summons to appear before and give evidence to the Culture, Media and Sport Select Committee.
On the few occasions he has been scrutinised by MPs, there have often been rhetorical fireworks and bad blood on both sides.