Boris Johnson should force an equalities minister to apologise or sack her, the former head of Downing Street’s race unit has urged, amid a deepening row over “the politics of division”. Simon Woolley said the prime minister must intervene in the row over Kemi Badenoch, after a damning leaked letter sent by a No 10 adviser suggested she may have broken the ministerial code when she publicly criticised a journalist on social media. The special adviser, Samuel Kasumu, also complained of unbearable tension within Downing Street over race and said he considered resigning over fears that the Conservatives were pursuing a “politics steeped in division”. Following frantic interventions from ministers and senior aides including the vaccines minister, Nadhim Zahawi, Kasumu retracted his resignation on Thursday night. Lord Woolley, who was chair of the race disparity unit in Whitehall until July, said the government must respond to Kasumu’s concerns that Badenoch may have broken ministerial rules when she publicly criticised Nadine White, a HuffPost journalist. Woolley told the Guardian: “I hope that Samuel’s bravery in calling this out provokes a true leadership response. Unless Kemi Badenoch offers a fulsome apology to Nadine White, her position appears to be untenable.” Rather than respond to a journalistic inquiry, in which White asked via email why Badenoch had not appeared in a video featuring other cross-party black MPs encouraging uptake of the Covid-19 vaccine, the minister provoked an outcry by accusing the journalist on Twitter of “sowing distrust” and “making up claims”. Woolley, who also heads Operation Black Vote, said the prime minister must act decisively to regain the trust of black, Asian and minority ethnic (BAME) communities and campaigners against racism. “This is a critical moment for the government,” he said. “A key black special adviser in No 10 Downing Street is deeply concerned about the politics of division. Samuel Kasumu is referring to the government’s strategy that pits poor white people against poor black people, for example, by rubbishing Black Lives Matter and arguing that class in the northern regions, a code for white working class, is a greater inequality than racism. “What Kasumu and others want is for the government to about-face and radically begin to acknowledge and deal with the deep-seated racial inequalities that have been laid bare by Covid-19, such as jobs, health and housing.” The minister for women and equalities, Liz Truss, gave a speech in December detailing how she would move focus away from “fashionable” issues of race and gender and towards economic inequalities. Kasumu, an entrepreneur who has worked as a political appointee for the government since being recruited under the premiership of Theresa May, offered his resignation to Johnson on Thursday morning. The behaviour of Badenoch, who used Twitter to publish emails from the HuffPost journalist, calling them “creepy and bizarre”, had been concerning, Kasumu wrote. In his resignation letter, Kasumu wrote that “more concerning than the act, was the lack of response internally”. “It was not OK or justifiable, but somehow nothing was said. I waited, and waited, for something from the senior leadership team to even point to an expected standard, but it did not materialise.” Badenoch has previously provoked controversy by saying it was illegal for schools to teach white privilege as uncontested fact, and by accusing the family of a 12-year-old boy arrested over a toy gun for “inflaming tensions” about police behaviour. No 10 initially defended Badenoch over her HuffPost response, but the BBC reported that the Cabinet Office was now understood to be looking into whether she had broken the ministerial code. On Friday afternoon the prime minister’s spokesperson denied that claim, saying: “There is no Cabinet Office investigation ongoing. The prime minister fully supports the important work the minister is doing to improve uptake of the vaccine amongst ethnic minority communities.” Kasumu said progress made by the Conservatives under David Cameron in 2015 in appealing to minority ethnic voters had been reversed. “I fear for what may become of the party in the future by choosing to pursue a politics steeped in division,” he wrote, adding: “The damage that is often caused by our actions is not much considered.” It is understood that tensions have been building within No 10 since Munira Mirza was asked in June to appoint members of the new government commission on racial inequalities. A key Johnson adviser, Mirza has previously cast doubt on the existence of institutional racism and condemned previous inquiries for fostering a “culture of grievance”. Tony Sewell, the commission’s chair, has in the past questioned the idea of institutional racism. He was forced to apologise in July for making homophobic comments after the former footballer Justin Fashanu disclosed he was gay in 1990. A Downing Street spokesperson said of Kasumu: “It would not be appropriate to comment on individual staff members. This government is committed to inclusion and bringing communities together, and is the most ethnically diverse in this country’s history. Last year we established a commission on race and ethnic disparities to examine and tackle inequality and discrimination wherever it is found. It is due to report shortly.”