British Prime Minister Boris Johnson's chief adviser Dominic Cummings is poised to quit by the end of the year, the BBC said, as a power struggle at the heart of government became public this week.

Cummings, a controversial figure and one of the architects of the 2016 Brexit campaign, told the BBC late Thursday recent speculation that he was threatening to resign were "invented".

But he reiterated comments made in a blogpost in January that he planned to make himself "largely redundant" within a year.

"My position hasn't changed since my January blog," Cummings told the British broadcaster.

The BBC quoted a senior source in Johnson's Downing Street office as saying the divisive top aide would leave his job by Christmas.

Cummings' reported imminent departure will coincide with the end of the Brexit transition period, with the country set to embark 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 things to shake things up and come along with ideas, he's actually been that person," he added.

"I've enjoyed working with him, we haven't always agreed over everything, that's how it should be."

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

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

Cain had been offered the post of Johnson's chief of staff but a subsequent backlash among some Conservatives and within the prime minister's inner circle instead led to him resigning.

Far less high-profile than Cummings, he will leave his post at the end of the year.

Cummings is a highly controversial figure best known for a scandal earlier this year when he made a cross-country journey during the coronavirus lockdown.

His actions prompted a furious public and political backlash, with the government's strict rules to curb the pandemic prohibiting leaving home except for exceptional reasons, but he refused to resign.

The top aide, who suffered from Covid-19 symptoms at the time while his wife had contracted the virus, claimed he had needed to make the journey to ensure he had childcare options for his four-year-old son.

An enigmatic figure with an unconventional dress style and combative approach to politics, Cummings has endeared himself to a segment of Britons who have developed a distaste for the ruling elite.

His role in spearheading the Brexit campaign was made into a TV film that further fed the mystique surrounding him.

But his reportedly controlling style and frequent clashes with colleagues are said to have led to significant tensions within Johnson's government, which has been increasingly faltering under the pressure of the pandemic.