Five per cent of UK adults do not believe the Holocaust took place and one in 12 believes its scale has been exaggerated, a survey has found.
The poll of more than 2,000 people was carried out by Opinion Matters for the Holocaust Memorial Day Trust (HMDT).
Almost two-thirds of respondents could not say how many Jews were murdered or “grossly” under-estimated the number.
On Sunday hundreds of thousands of people will gather across the country to mark Holocaust Memorial Day.
Survivors, politicians and members of the public will remember the six million Jewish victims of the Holocaust, while also marking the 25th anniversary of the Rwandan genocide and 40 years since the end of the genocide in Cambodia.
More than 11,000 activities are expected to take place, including a national commemorative ceremony in Westminster, as well as other events in Scotland, Wales and Northern Ireland.
Holocaust survivor Steven Frank, who was one of 93 children to survive the Theresienstadt camp in Czechoslovakia along with his two brothers, said the survey’s findings were “terribly worrying”.
His father, who helped hide Jews as part of the Dutch resistance, was arrested in Amsterdam and gassed at Auschwitz, in Nazi-occupied Poland, in January 1943.
Mr Frank, who now lives in Hertfordshire, said he was “surprised” by the number of people who do not believe the Holocaust took place.
The 83-year-old 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.”
He added: “Education is so important. If we ignore the past, I fear history will repeat itself.”
The HMDT’s chief executive Olivia Marks-Woldman said: “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.”
She added that the rise in reported hate crime in the UK and the risk of genocide in ongoing international conflicts meant “we cannot be complacent”.
Karen Pollock, chief executive of the Holocaust Educational Trust, said: “One person questioning the truth of the Holocaust is one too many.
“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.”