Downing Street’s most senior Black adviser has resigned amid a backlash over the government’s report into racism in the UK, it has been reported.

Samuel Kasumu, Boris Johnson’s adviser on ethnic minorities, quit on Tuesday morning, POLITICO first reported.

The report by the Commission on Race and Ethnic Disparities, which was set up after Black Lives Matter anti-racism protests across the country last summer – triggered by the killing of George Floyd in the US – argued Britain is no longer a country where the “system is deliberately rigged against ethnic minorities”.

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

Anti-racism campaigners have branded the commission a “whitewash”, while unions said the report was “deeply cynical” and denied the experiences of Black and minority ethnic workers. “We are being gaslighted,” said Labour’s shadow justice secretary David Lammy.

Boris Johnson thanked Kasumu for his work on encouraging vaccine take-up among “more hesitant groups and communities”, when asked about the adviser’s resignation on Thursday.

During a visit to Middlesbrough, Johnson told reporters: “I worked very closely with Samuel in the last year or so and he’s done some great stuff.

“I thank him very much, particularly on helping to encourage vaccine take-up amongst more hesitant groups and communities. And, actually, we’re seeing some real success there.

“It is true that different groups have been coming forward at different paces, everybody is increasing their take-ups, so I thank him very much for that.”

Labour MP Dawn Butler, a former shadow equalities secretary, said Kasumu had made a “principled” decision to quit.

“Samuel when you’re free I would like to buy you a drink or two,” she added.

Asked about Kasumu’s resignation, skills minister Gillian Keegan told Times Radio: “I don’t even know who he is.”

Kasumu almost quit government in February, after equalities minister Kemi Badenoch attacked a HuffPost UK journalist on Twitter.

But he was ultimately persuaded to stay following talks with vaccine minister Nadhim Zahawi.

In his resignation letter, seen by the BBC, he warned that progress made by the Conservative Party under David Cameron in 2015 with Black and Asian communities had been destroyed.

“The damage that is often caused by our actions is not much considered,” he said.