The Countdown co-presenter and longtime critic of Mr Corbyn on Tuesday posted a selfie, which showed the 33-year-old wearing a t-shirt featuring an image of the Labour leader being led away by police during an anti-apartheid rally in 1984.
In it, Mr Corbyn is wearing a sign edited to read, “Jeremy Corbyn is a racist endeavour”. The original message actually said: “Defend the right to demonstrate against apartheid. Join this picket.”
Captioning the photo in reference to ITV’s leadership debate earlier this week, Riley wrote: “I didn’t feel comfortable knowing my workplace was to be full of racists tonight. I don’t endorse Boris, but I do endorse #NeverCorbyn.”
Many Labour supporters reacted angrily, accusing the TV star, who identifies as Jewish, of insensitivity by altering an anti-racism message to accuse Mr Corbyn of racism.
"Absolutely staggering ignorance from Rachel Riley," one tweeted, while another said: "She calls out antisemitism but is utterly blind to the black struggle or she simply doesn't care."
Chardine Taylor Stone, an LGBTQ activist, tweeted: “Rachel Riley needs to be cancelled. YOU DO NOT treat apartheid as joke to make a statement. I don’t care who is holding the placard or what statement she thinks she is making here.”
Riley, who has been contacted for comment, on Thursday morning tweeted a series of news articles suggesting former South African president Nelson Mandela had "snubbed" Mr Corbyn because he was considered a "hindrance" to the anti-apartheid movement.
The claim has since been largely debunked. In fact, it was the City of London Anti-Apartheid Group, which was behind the South African embassy protests, that received no reply when it wrote to Mr Mandela in 1990 to gain his retrospective approval for the picket.
Although Mr Corbyn has proudly acknowledged his part in the protests, nearly every Labour MP at the time also did no, according to the New Statesman.
Her attack on Mr Corbyn came days after a former Labour MP called on the equalities watchdog to publish its findings of a major investigation into allegations of antisemitism in the party.
Luciana Berger, who is now contesting Finchley and Golders Green for the Liberal Democrats, accused the Labour leader of having “failed” to properly deal with anti-Jewish racism.
Speaking as the Lib Dems unveiled their election promises on equalities and human rights issues, Ms Berger said the Equality and Human Rights Commission (EHRC) should publish its work before voters head to the polls on 12 December.
The EHRC has not revealed any timeframe for the Labour investigation, but the organisation told The Independent it was still ongoing and will not be concluded before the end of 2019.
Launched in May, the investigation seeks to determine “whether unlawful acts have been committed by the party” and “whether the party has responded to complaints of unlawful acts in a lawful, efficient and effective manner”.
The Labour Party declined to comment.
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