Dominic Cummings, the controversial mastermind of the 2016 campaign for Britain to leave the European Union, will quit as Prime Minister Boris Johnson's top aide once Brexit is completed later this year, reports said Friday.

Johnson's chief adviser, a divisive figure who has carved out a uniquely powerful role in government, told the BBC late Thursday recent speculation that he was threatening to resign were "invented".

But amid a power struggle within Downing Street that has spilled into public view this week, Cummings reiterated comments made in a blogpost in January that he would make himself "largely redundant" within a year.

"My position hasn't changed since my January blog," Cummings told the broadcaster, which quoted a senior government source as saying he would leave his job by Christmas.

Cummings departure then would coincide with the end of the Brexit transition period, when Britain embarks on life outside of European Union rules and regulations on January 1.

"He'll be missed but then again, we'll be moving into a different phase," Transport Secretary Grant Shapps told Sky News on Friday.

"In any government you require people who are going to shake things up and come along with ideas, he's actually been that person."

Cummings, an enigmatic figure with an unconventional dress style and combative approach to politics, was appointed chief adviser by Johnson when he took power in July 2019.

He helped to secure a thumping election victory last December, but his reportedly controlling style and frequent clashes with colleagues are said to have led to persistent tensions.

He courted damaging controversy earlier this year after making a cross-country journey during the coronavirus lockdown which appeared to breach strict regulations he had helped to design.

Cummings, who had Covid-19 symptoms at the time while his wife had contracted the virus, claimed it was necessary to ensure he had childcare options for their young son.

His actions prompted a furious public and a political backlash but he refused to resign and Johnson stood by him at considerable political cost.

Cummings' reported imminent departure will coincide with the end of the Brexit transition period
Cummings' reported imminent departure will coincide with the end of the Brexit transition period AFP / DANIEL LEAL-OLIVAS

Cummings has since helped to spearhead the government's response to the pandemic, and is thought to be the architect of Johnson's "moonshot" plan for millions of daily tests to allow those without coronavirus to circulate freely.

Downing Street has faced sustained criticism over its centralised handling of the crisis.

Britain has seen more than 50,000 deaths in the outbreak -- the highest in Europe -- and a second wave shows few signs of abating.

Johnson's response to the increase in cases has been to impose a four-week partial lockdown in England, angering many of his own Conservative MPs.

One Tory veteran, Bernard Jenkin, said Cummings' exit would be a chance to restore "respect, integrity and trust" between MPs and Downing Street, which have been "lacking in recent months".

"It's an opportunity to reset how the Government operates and to emphasise some values about what we want to project as a Conservative Party in government," he told BBC radio.

The news comes just a day after the resignation of Johnson's communications director Lee Cain, a close Cummings ally who also worked on the "Leave" side of the 2016 referendum.

The less high-profile, Cain -- a former tabloid journalist who once dressed as a chicken to goad Johnson's predecessor David Cameron -- had been offered the post of chief of staff.

But a subsequent backlash among some Conservatives and within the prime minister's inner circle instead led to him to quit.

The departure of both men, alongside the launch of daily televised press briefings, could herald a shift in tone and strategy from Johnson's government.

Lawmaker David Lammy, from the main opposition Labour party, said the departures were "like rats fleeing a sinking ship".

"His (Cumming's) legacy is one of bullying, deception, hypocrisy and hubris," he added.