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

Downing Street said Samuel Kasumu will remain in post until May and he had planned his departure for several months.

His exit is not linked to the landmark report by the Commission on Race and Ethnic Disparities (Cred), which faced heavy criticism over its findings, although its timing is uncomfortable for the Government.

A No 10 spokesman said Mr Kasumu has played an “incredibly valuable role” during his time as a special adviser.

“As he previously set out, he will be leaving government in May – this has been his plan for several months and has not changed.

“Any suggestion that this decision has been made this week or that this is linked to the Cred report is completely inaccurate.”

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 Cred report was published on Wednesday and has faced heavy criticism.

Commission chairman Dr Tony Sewell said his team 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.

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.

In response to the criticism, Dr Sewell said to suggest it was “trying to downplay the evil of the slave trade” is “absurd”.

“It is both ridiculous and offensive to each and every commissioner,” he said.

“The report merely says that, in the face of the inhumanity of slavery, African people preserved their humanity and culture.”

The report proposed 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 his foreword to the report, Dr Sewell 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.”