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.
Today 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.
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.”