Dominic Cummings was pictured exiting the prime minister’s 10 Downing Street office through the front door.
British media reported he had left for good rather than staying on as Johnson’s adviser until the end of the year, when he was expected to give up his post.
The prime minister's office hasn't confirmed Cummings' departure.
Nicknamed “Boris' brain,'' Cummings has also been the target of complaints from senior members of Johnson’s Conservative Party, who say that unelected advisers in the prime minister’s Downing Street office were effectively running the government, sidelining ministers and Parliament.
Cummings’ departure is an opportunity to reshuffle the government and bring in more experienced ministers, Jenkin said.