Greta Thunberg, Tracey Emin and Gary Lineker are among those to sign an open letter in support of Southbank Centre chair Misan Harriman, after what they call a “dishonest smear campaign” by media outlets who accused him of promoting Golders Green attack “conspiracies” and comparing Reform voters to Nazis.

Harriman, who has been chair of the Southbank Centre’s board of governors since 2021, was accused by the Telegraph of sharing a social media post that contained a “conspiracy” about the Golders Green attack because it questioned the amount of coverage given to the Muslim victim, Ishmail Hussein.

Critics of Harriman said the repost risked minimising the antisemitic nature of the attack. David Taylor, the Labour MP for Hemel Hempstead, said: “These posts are not only incredibly inappropriate for the chair of a charity board, but for anyone in the public eye.”

The Telegraph then ran a story with the headline “Southbank Centre chief ‘compares Reform victory to Holocaust’” after the activist quoted Susan Sontag in a video giving his thoughts after Reform’s historic local election results.

He said: “She said when thinking about the Holocaust, 10% of people in any population are cruel no matter what, and 10% is merciful no matter what and the other – this is important – the other remaining 80% could be moved in either direction.

“It’s such a profound way to look at us. In the context of yesterday’s election result it is something which I think is really topical.”

Karen Pollock, the chief executive of Holocaust Educational Trust, also criticised Harriman, asking: “How on earth could yesterday’s election results ever be comparable to the Holocaust?”

Robert Jenrick, the Reform MP, said the post was “disgusting” and called for him to be removed from his position at the Southbank Centre. He wrote: “This crass moron should be nowhere near a taxpayer-funded organisation.”

Amid widespread coverage of Harriman’s comments, the letter reflects concern that public figures are being silenced for speaking out at a moment of heightened tension over antisemitism. But critics argue that Harriman’s role leading a major publicly funded institution makes the scrutiny not only legitimate but necessary.

“The purpose of the smear campaign, which we repeat is entirely without foundation in fact, is to traduce and marginalise Misan,” reads the letter. “And it is intended to send a message to others that if they speak out, they will be subject to harassment and threats.

More than 245 people signed the letter, including Riz Ahmed and David Oyelowo, which said that “trying to silence responsible critics of Israel by smearing them as antisemitic does not protect Britain’s Jewish community”.

Several Jewish cultural figures, including Pulitzer prize-winning Sontag biographer Benjamin Moser, actor Morgan Spector and the photographer Jillian Edelstein, signed the letter.

Harriman told the Guardian: “We have reached the point where truth itself is being crushed by the very institutions that are supposed to uphold it. I will never whisper about the oppressed. I stand with truth, I stand by my right to use my voice to help others.”

The letter comes after 53,000 people backed a campaign to lobby the press regulator Ipso about the coverage, which is more than double the number of people who complained about Jeremy Clarkson’s 2022 column where he said he wanted the Duchess of Sussex “paraded naked through the streets of every town in Britain”.

Harriman has built a large online following after emerging as a photographer during the Black Lives Matter protests and is the subject of a forthcoming documentary about his work made by Bafta-winning director Andy Mundy-Castle.

He was chosen by the Duke and Duchess of Sussex to shoot their portrait, while Harriman has worked with children who have fled Gaza, giving them equipment to shoot their own images. He is also an ambassador for Save the Children.

A spokesperson from the Southbank Centre said the institution was an “inclusive and welcoming place for everyone including our artists, audience and all colleagues.

“The Southbank Centre condemns all forms of antisemitism, hatred and discrimination. All Southbank Centre board members, including the chair, have the right to exercise their freedom of expression within the law. The personal views of individual members of our board do not represent the views of the Southbank Centre and in no way affect our programming nor the welcome that we extend to all.”

Keir Starmer will not lead his party into the next general election, Labour-supporting unions have predicted, in an intervention that threatens to further destabilise the prime minister after a damaging few days.

The 11 Labour-affiliated unions – which include Unite, Unison and the GMB – are expected to issue a joint statement on Wednesday saying “at some stage” the party will have to put a plan in place to elect a new leader.

At a private meeting on Tuesday, the unions were divided over whether to call for Starmer to set out a timetable for his departure, with one source telling the Guardian there had been a “big fight” among union officials.

However, they are understood to have agreed to issue a statement saying they expect there to be a change of leadership, despite GMB and Community arguing it was not in the unions’ best interests to get involved in leadership wrangling.

In a leaked copy of the statement, the unions said it was clear to them that Labour “cannot continue on its current path”, and despite some progress it was not doing enough to deliver the change people voted for at the last election.

They urged the party leadership to focus on the “fundamental change of direction on economic policy and political strategy” they said working people needed, rather than the “personalities and unfolding political drama” at Westminster.

Starmer was increasingly confident that he had seen off the immediate threat to his job on Tuesday after a challenge from Wes Streeting failed to materialise despite several of the health secretary’s allies quitting the government.

But the prime minister’s fragile authority has been weakened by the resignation of four ministers – three of them close allies of Streeting – in what appeared to be an orchestrated move. More than 90 Labour MPs have also called for him to go since the weekend.

Starmer, who told his cabinet he would fight on as prime minister after a turbulent few days, was hoping that his second king’s speech on Wednesday would be another reset moment for the government to help unite his deeply divided party.

While he appears to have survived for now, even his most loyal ministers have privately acknowledged that he is unlikely to take Labour into the next election, unless he can dramatically turn round his and the government’s fortunes.

In their draft statement, which is due to be released on Wednesday, the union general secretaries wrote: “Labour’s affiliated unions have been clear that Labour cannot continue on its current path.

“Whilst we recognise progress has been made, such as aspects of the Employment Rights Act and the increase in the minimum wage, the results at the election last week were devastating.

“Labour is not doing enough to deliver the change that working people voted for at the general election. Our focus is on the fundamental change of direction on economic policy and political strategy that unions have been clear is needed, and not on the personalities and unfolding political drama in Westminster.”

But they added: “It’s clear that the prime minister will not lead Labour into the next election, and at some stage a plan will have to be put in place for the election of a new Leader.

“This is a point where the future of the party we founded will be debated and determined – and we are working closely as unions to shape a shared vision on policy, political strategy and economic policy that will reorient Labour back to working people, so Labour do what it was elected to do: govern in the interests of workers.”

Union officials had been frustrated when Downing Street postponed a meeting of the Trade Union and Labour Party Liaison Organisation [TULO] due on Tuesday, but agreed to meet for talks anyway.

The Labour leadership has had an increasingly tense relationship with the unions – which help to fund the party – since coming to power, despite big wins for unions on issues including workers’ rights and the minimum wage.

Some union leaders have urged Starmer to quit, with Unite’s Sharon Graham saying the “writing is on the wall” for the prime minister after last week’s election disaster. Others have urged the party to focus on its plan to change the country rather than arguing about the leadership.

In the US, where it is available, the price is cheaper than current injectables, at around $149 per month for the lowest dose, compared with over $1,000 a month for some GLP-1 injections (although US President Donald Trump has announced deals that aim to lower the cost of popular weight-loss drugs).