The government is being urged to fund a memorial to remember the victims of the Transatlantic slave trade.
The charity Memorial 2007 has planning permission for a sculpture in London's Hyde Park, but has been unable to raise £4m to fund it.
Consent granted in 2016 for the memorial in the Rose Gardens is due to expire on 7 November unless the charity can secure an extension.
It said millions had been given to for a Holocaust memorial "but not to us".
More than 53,000 people have signed a petition asking the government to build a major memorial to remember the victims of the British slave trade.
Britain became the world's leading slave-trading nation with Liverpool port its largest handler of slaves, followed by Bristol, Glasgow and London.
Millions of Africans were shipped and sold in the 17th and 18th Centuries in horrendous conditions as merchants profited.
Teacher Oku Ekpenyon has campaigned since 2002 for a dedicated memorial after being asked by a pupil of African heritage at the Tower of London, "Miss, where is our history?"
"The government has supported over the years various memorials financially; why not support us?" she said.
The charity, supported only by volunteers, has raised £100,000 through small donations.
Ms Ekpenyon said the subject matter was "too sensitive for some people to countenance", with big donors or corporations veering away from supporting them.
Toyin Agbetu, founder of Ligali, a pan-African human rights organisation, said: " Memorials are such an important part of the national narrative.
"If we've got a war memorial to animals who died (in Hyde Park), we certainly can have a memorial for some of those ancestors of many of those people in this nation who actually helped build it up."
The government said it "carefully considered" each request for funding.
It said: "We are supportive of the aims of the monument and the organisation. The suffering caused by slavery and the slave trade are among the most dishonourable and abhorrent chapters in human history."