According to the Politico media outlet, Kasumu, who is Johnson’s most senior black adviser, had informed colleagues of his decision on Tuesday morning, a day before the publication of the controversial report, but he will continue in his post until May.
On Wednesday, the Independent Commission on Race and Ethnic Disparities, which was appointed by Johnson following last summer's Black Lives Matter protests, concluded that the UK is no longer a country "where the system is deliberately rigged against ethnic minorities."
The review also argued that although the UK cannot be considered "a post racial society," it had become a "beacon to the world" in successfully creating a multi-ethnic society, adding that most of the disparities which some attribute to racial discrimination, "often do not have their origins in racism."
The 264-page report sparked a barrage of criticism from civil rights activists and lawmakers from the Labour Party, among others.
"Institutionally, we are still racist, and for a government-appointed commission to look into (institutional) racism, to deny its existence is deeply, deeply worrying," Halima Begum, the chief executive of the leading race equality think tank Runnymede Trust, said.
Labour lawmaker and shadow foreign secretary Lisa Nandy called the review "disappointing" and accused the government of trying to downplay the structural nature of racism.
"I think [the report] is very disappointing. On the one hand, the report seems to recognize that there are huge problems that still affect black and minority ethnic people in this country every day, but in the other hand, it seeks to downplay the structural nature of some of those challenges," she said.
Nandy argued that Black Caribbean children are 3.5 times more likely to be excluded from schools than their white counterparts, while black people are four times more likely to be arrested than white people.
Her fellow party member, lawmaker David Lammy, whose 2017 review into racial disparities in the criminal justice system is still awaiting implementation, called the race commission’s conclusions "insult to anybody and everybody across this country who experiences institutional racism."
"This report could have been a turning point and a moment to come together. Instead, it has chosen to divide us once more and keep us debating the existence of racism rather than doing anything about it," the shadow justice secretary told LBC radio station.
Reacting to the review, Johnson announced in a statement that his government will consider its recommendations and assess the implications for future government policy.
The 24 recommendations made in the report include phasing in extended school days to help pupil catch up on missed learning during the pandemic, starting with disadvantaged areas, and discontinuing the use of the acronym BAME (Black, Asian and Minority Ethnic) as differences between groups are as important as what they have in common.
Sputnik has approached the Prime Minister’s office for comments on local media reports about Kasumu’s resignation but has not received any response so far.