Holocaust experts - including shoah survivor Sir Ben Helfgott - have called on Facebook to delete flagrantly antisemitic posts after it refused to do so.

In an open letter to Facebook boss Mark Zuckerberg, the heads of 24 institutions warn the social media giant that it is allowing "complete and utter falsehoods" about the Holocaust to go "systematically unchecked".

"Facebook must not allow complete and utter falsehoods about the Holocaust, and about the Jewish people, to go systematically unchecked. Virulent antisemitism is a proven pathway that leads from rhetorical hatred to actions of violence," the letter, published on Tuesday morning.

"Virulent antisemitism is a proven pathway that leads from rhetorical hatred to actions of violence. Freedom of speech laws are not a reason to do nothing — inaction is always the opportunity for evil to flourish."

As well as Sir Ben, signatories include Henry Grunwald, chairman of the UK National Holocaust Centre and Museum, UK Holocaust Memorial Day Trust chair Laura Marks, Holocaust Educational Trust chair Paul Phillips and National Holocaust Centre and Museum President Dr James Smith.

The letter follows a Times report about how Facebook failed to remove a post that said Jewish people were "barbaric and unsanitary".

It also follows the huge controversy Mr Zuckerberg generated after he suggested Holocaust denial was not something Facebook would automatically remove, saying shoah deniers may simply be "getting a few things wrong".

"All genocide starts with a distortion of the truth," the open letter says. "No society can afford to ignore, hide or bury antisemitism if it wishes to remain civilised. History proves that it is the canary in the coal mine; the first unravelling of a society’s moral fabric.

"During World War II, it was the first rung on the ladder of prejudice and discrimination that led to genocide — first against Jews and then other groups including political opponents, homosexuals, Roma and Sinti people. Hatred of one group within society leads to hatred of others."

They offer an action plan for tackling Holocaust denial, including "proven educational resources in multiple languages, ready for digital deployment...  cost-free professional development programs for educators" that teach how to "tackle hate and prejudice, and to teach empathy, understanding and respect".

"We have thousands of shareable stories that reveal the personal dimension of hate-based violence and the inspiring people who have stood up against it," the open letter says.

"We can together help those who are “getting a few things wrong” to get a few things right."

A Facebook spokesperson told the Times: “We take the issue of antisemitism and any form of hate speech incredibly seriously, and find it deeply offensive.”