Dominic Cummings is to leave his role as the prime minister's chief adviser by the end of the year, Sky News understands.
A Downing Street source said the controversial aide will leave Number 10, having been one of Boris Johnson's first appointments when he became prime minister in July last year.
In a blog post in January, the PM's top aide said he planned to leave his role by the end of the year, but his departure will now come after a bitter power struggle behind the scenes in Downing Street - something that is likely to have hastened his decision.
The row came to light with the resignation of his close ally Lee Cain as the prime minister's director of communications.
Mr Cummings and Mr Cain both worked with Mr Johnson as part of the Vote Leave campaign during the 2016 EU referendum.
They are widely regarded to have since led a Vote Leave faction of colleagues in Number 10 and other parts of government.
But their departures follow the appointment of former journalist Allegra Stratton as Downing Street's new TV spokeswoman, which was said to have been opposed by Mr Cain.
Mr Cummings - once reportedly described by former prime minister David Cameron as a "career psychopath" - is a former Conservative Party director of strategy and ex-aide to senior cabinet minister Michael Gove.
He was credited as one of the most influential figures behind Vote Leave's success under its "Take Back Control" slogan.
As part of Mr Johnson's top team, Mr Cummings helped secure last year's thumping general election victory for the Conservative Party.
He was said to have subsequently focused on projects such as setting up an advanced defence projects research agency and reforming government procurement processes.
Mr Cummings gained national notoriety this year when he was accused of breaching lockdown rules by making a trip from London to the North East, at a time when he feared he could have contracted coronavirus.
He also admitted to having driven to Barnard Castle as a means of testing his eyesight.
Despite huge pressure for Mr Johnson to sack his senior aide, the prime minister stood by Mr Cummings.
During his time in Number 10, Mr Cummings called for "misfits and weirdos" to apply for jobs in Downing Street.
One of those subsequently employed was soon forced to resign following the emergence of his past online comments.