Boris Johnson’s most senior black adviser has resigned as ministers face a backlash after a Government-backed review said Britain is no longer a country where the “system is deliberately rigged against ethnic minorities”.

Samuel Kasumu has quit his role as a special adviser to the Prime Minister on civil society but will stay in post until May to continue work on improving vaccine uptake in minority groups, Politico reported.

The timing of Mr Kasumu’s departure comes after the landmark report by the Commission on Race and Ethnic Disparities (Cred) faced heavy criticism over its findings, with claims that it is culturally deaf, out of step with public opinion, and “steeped in denial”.

Its chairman said it had found no evidence of “institutional racism”, and the report criticised the way the term has been applied, saying it should not be used as a “catch-all” phrase for any microaggression.

Downing Street sources insisted his departure was “absolutely nothing to do with the report”.

Government minister Gillian Keegan appeared unaware of Mr Kasumu’s departure, telling Times Radio: “I don’t even know who he is.”

Politico said Mr Kasumu notified the Prime Minister’s chief of staff, Dan Rosenfield, of his decision to quit his job – which paid up to £75,000 – last week.

He has reportedly been unhappy in Government for some time, with a resignation letter drafted – but then retracted – in February.

In the letter, which was obtained by the BBC, Mr Kasumu accused the Conservative Party of pursuing “a politics steeped in division” and suggested Equalities Minister Kemi Badenoch may have broken the ministerial code in her public spat with a journalist.

Shadow women and equalities secretary Marsha de Cordova said: “To have your most senior adviser on ethnic minorities quit as you publish a so-called landmark report on race in the UK is telling of how far removed the Tories are from the everyday lived experiences of black, Asian and ethnic minority people.

“Their divisive report appears to glorify slavery and suggests that institutional racism does not exist, despite the evidence to the contrary. It is no wonder they are losing the expertise from their team.”

The commission said geography, family influence, socio-economic background, culture and religion all affect life chances more than racism.

Its findings have been described as insulting and divisive, and the chairman of the review has been accused of putting a “positive spin on slavery and empire” when explaining its recommendation on teaching history in schools.

The report proposes a Making Of Modern Britain teaching resource to “tell the multiple, nuanced stories of the contributions made by different groups that have made this country the one it is today”.

In commission chairman Tony Sewell’s foreword to the report, he said the recommendation is the body’s response to “negative calls for ‘decolonising’ the curriculum”.

He wrote that the resource should look at the influence of the UK during its Empire period and how “Britishness influenced the Commonwealth” and how local communities influenced “modern Britain”.

He added: “There is a new story about the Caribbean experience which speaks to the slave period not only being about profit and suffering but how culturally African people transformed themselves into a remodelled African/Britain.”

Highlighting the passage on Twitter, Ms de Cordova said it was “one of the worst bits” of the report which was “putting a positive spin on slavery and empire”.

Halima Begum, chief executive of race equality think tank the Runnymede Trust, said: “Comments about the slave trade being a Caribbean experience, as though it’s some kind of holiday… this is how deafening it is, cultural deafness, it’s completely out of kilter with where British society is, I believe.”