One in 20 UK adults does not believe the Holocaust took place, a survey suggests, while one in 12 believes its scale has been exaggerated.

Almost two-thirds of respondents (64%) either could not say how many Jews were murdered or “grossly” under-estimated the number, a survey of more than 2,000 people by the Holocaust Memorial Day Trust (HMDT) found.

On Sunday, hundreds of thousands of people, including survivors, politicians and members of the public, will gather to mark Holocaust Memorial Day and remember its six million Jewish victims.

More than 11,000 activities are expected to take place, while ceremonies will be held in London, Scotland, Wales and Northern Ireland.

A national commemorative ceremony will take place in Westminster to mark the day, which is also acknowledging the 25th anniversary of the genocide in Rwanda and 40 years since the end of the genocide in Cambodia.

Holocaust survivor Steven Frank, who was one of 93 children who survived the Theresienstadt camp in Czechoslovakia along with his two brothers, said the figures were “terribly worrying”.

His father, who helped hide Jews as part of the Dutch Resistance, was arrested in Amsterdam and taken to Auschwitz in Nazi-occupied Poland, where he was gassed on January 21 1943.

Mr Frank, 83, said he was “surprised” that the survey found as many as one in 20 people still did not believe the Holocaust took place.

He said: “In my experience, people don’t have a solid understanding of what happened during the Holocaust and that’s one of the reasons I am so committed to sharing what happened to me.

“At one of my talks, I met someone who said the Holocaust didn’t happen.

The only way to fight this kind of denial and anti-Semitism is with the truth – I tell people what happened, what I saw and what I experienced.

“Education is so important. If we ignore the past, I fear history will repeat itself.”

HMDT chief executive Olivia Marks-Woldman said: “The Holocaust threatened the fabric of civilisation and has implications for us all.

Such widespread ignorance and even denial is shocking.

“Without a basic understanding of this recent history, we are in danger of failing to learn where a lack of respect for difference and hostility to others can ultimately lead.

“With a rise in reported hate crime in the UK and ongoing international conflicts with a risk of genocide, our world can feel fragile and vulnerable. We cannot be complacent.”

Karen Pollock, chief executive of the Holocaust Educational Trust, said:

“We know that education is vital in the fight against ignorance and hate.

“Whatever the statistics, one person questioning the truth of the Holocaust is one too many and so it is up to us to redouble our efforts to ensure future generations know that it did happen and become witnesses to one of the darkest episodes in our history.”

Communities Secretary James Brokenshire, who will be speaking at Sunday’s event, said: “It remains essential now as ever to remember the Holocaust, to understand why it happened and to learn the stark lessons it gives.

“We must never forget where hatred and bigotry can lead.”

He added: “This Government is clear that anti-Semitism has no place in our society and we all have a role to play in confronting hatred and extremism whenever and wherever it occurs.”

German-Israeli satirist Shahak Shapira, who set up a website shaming selfie-takers at Berlin’s Holocaust Memorial, says he has halted the project for now after a dozen people apologised for their disrespect.

His “Yolocaust.de” website had combined selfies, often with the participants grinning or striking poses, taken at the memorial with graphic images from Nazi concentration camps, including piles of bodies.

“I’m watching you. Stop doing it,” Shapira told Reuters Television.

The memorial, located near the Brandenburg Gate, comprises 2,711 tombstone-like slabs of granite of varying heights. It is often used by visitors for picnics, yoga and other activities that Shapira said he found troubling.

About 2.5 million people had visited his website, he said.

All 12 people whose selfies he used had contacted him and apologised within a week of the images first being uploaded and most had now removed the inappropriate photos from their private websites.

Such photographs were also common at other sites including the Auschwitz and Treblinka death camps, he said.

“It’s about fighting ignorance, making people realise where they are, what this place stands for,” said Shapira, who lost half his family in the Nazi genocide.

Peter Eisenman, the U.S. architect who designed the memorial, said he loved the fact that people sunbathed or picknicked there.

“It is not the camp itself. It is not the sacred ground. It is a ground of remembrance and you can choose to remember in many ways, or not remember,” Eisenman said. “It’s become part of the fabric of the city.”

More worrying, he said, was the growing power of the German right-wing and that such a memorial might not be approved in the first place now.

Friday is an international memorial day for the victims of the genocide that resulted in the death of an estimated 6 million Jews, 2 million Sinti and Roma people, 250,000 mentally and physically disabled people, and 9,000 homosexuals.

The ceremonies occur on the 72nd anniversary of the liberation of the Auschwitz death camp.