Boris Johnson has been urged to reject the findings of a “whitewashed” government-backed report which claimed institutional racism does not exist in the UK.

Windrush campaigners and victims were among those who handed an open letter signed by 21,882 people to Downing Street on Friday urging the prime minister to withdraw the report.

Organisations including the Runnymede Trust, a leading anti-racism think-tank, the National Education Union (NEU) and Liberty backed the letter, claiming the report’s findings have led to “public incredulity and national indignation” for failing to address institutional racism.

The letter calls on the PM to instead implement the recommendations of the long-standing Macpherson, Lammy, Marmot and Williams reviews into racism.

The letter also has the support of leading figures, including the former chair of the government’s race disparity unit advisory group, Lord Simon Woolley.

Johnson last week tried to distance himself from the controversial commission on race and ethnic disparities report as his most senior Black adviser, Samuel Kasumu, quit the government.

The PM stressed: “I don’t say the government is going to agree with absolutely everything in it”.

But he has faced allegations of attempting to steer the review’s findings by giving control of it to his top policy adviser Munira Mirza, who has previously accused an “anti-racism lobby” of fostering a “culture of grievance”.

The government was also criticised after charity boss Tony Sewell was made chair of the review. He had previously claimed evidence of institutional racism was “flimsy”.

Windrush campaigner Patrick Vernon, who was among those handing the letter in to Downing Street on Friday, said: “We hope the prime minister seriously rejects the commission’s so-called ‘race report’ and instead makes a commitment to support existing race equality legislation and best practice in promoting the public sector equality duty. 

“It is important that he acknowledges the lived experiences of minority communities and especially those experiencing the hostile environment and the ongoing impact of the Windrush scandal, where institutionalised racism is still alive and kicking in Britain today.”

Kevin Courtney, joint general secretary of the NEU said: “As we approach the anniversary of Stephen Lawrence’s murder, we are disappointed that so much public money has been spent on this worse than useless report. 

“It lurches back to the days before the Macpherson inquiry when a culture of denial of institutional racism was prevalent. 

“The progress made since then is now in danger of being squandered and to paraphrase Stephen’s mother, Doreen Lawrence, it may give the green light to racism.

“The NEU rejects this report and calls on the government to reject it too.”

Halima Begum, CEO of the Runnymede Trust said: “We want to work with the government to ensure that the mood across the country on anti-racism is reflected and acted upon.

“In this instance, we are unable to accept the findings of the report. 

“The current national outrage supports our reservations and challenges with the report. 

“However, we hope that the PM will see this as an opportunity to make a decision on his anti-racism stance, which he clearly stated after the onset of Black Lives Matter last summer.

“It is not individual acts of racism (as bad as they are) that hurts the collective prospects of BME communities in this country. 

“It is institutional racism that produces the disproportionate outcomes in our communities. We hope that the prime minister will work with us to tackle this.”