Dominic Cummings, British Prime Minister Boris Johnson's chief adviser, has quit his role at Downing Street after an internal battle over who should be Mr Johnson's chief of staff.

Mr Cummings had originally said he would leave his position at the heart of No 10 when Brexit is done at year-end, but was seen leaving Downing Street on Friday afternoon carrying a cardboard moving box.

It was later confirmed he had left his role effectively immediately.

Mr Cummings, who masterminded the 2016 Brexit referendum's Leave campaign and Mr Johnson's 2019 landslide election bid, told the BBC on Thursday that his position had not changed since January when he said he wanted to be largely redundant by the end of this year.

It was not made immediately clear why his departure was expedited to Friday.

Transport Secretary Grant Shapps told Sky that Mr Cummings had planned not to stay in his position as Mr Johnson's senior adviser for much more than a year, once Britain left informal membership of the European Union.

"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'd worked on, of course which was Brexit, coming to an end at the end of the transition period which is December 31," Mr Shapps said.

The exit of one of Mr Johnson's closest allies marks one of the most significant changes to the Prime Minister's inner circle to date: Mr Cummings was cast by some of Mr Johnson's "brain" — a figure who wielded significant influence.

A committed Brexiteer who scorned the British political establishment and hurled barbs at reporters and cabinet ministers alike, Mr Cummings was cast in the Spitting Image satirical puppet show as an alien who repeatedly threatened Mr Johnson with resignation — and sometimes asked to eat his child.

The BBC cited an unidentified senior Downing Street source as saying that Mr Cummings "jumped because otherwise he would be pushed soon".

The day before his departure Mr Cummings told the BBC that "rumours of me threatening to resign are invented, rumours of me asking others to resign are invented".

Mr Shapps said Mr Cummings would be missed as it was good to have someone in government to shake things up.

As Mr Johnson negotiates the final stages of a Brexit trade deal and grapples with a second wave of the novel coronavirus outbreak, Downing Street has been gripped by an internal battle over who should become Mr Johnson's chief of staff.

Mr Johnson's director of communications, Lee Cain, resigned on Wednesday (local time).

Mr Cain, another Brexit supporter, was a close ally of Mr Cummings and worked with him at the Vote Leave campaign.

Reuters