Joan Salter's message to young people is that "even in the most terrible times, there were still individuals willing to stand up and say it [the Shoah] was wrong. When your life is in danger, it's hard to be the first one to stand up. We must not underestimate the courage that's needed."

London-based Mrs Salter was speaking before recounting her family's wartime experiences to Wednesday's national Holocaust Memorial Day ceremony at Guildhall in central London.

Her family's journey began when she was three months old. It took them through Belgium, France, Spain and Portugal, from where she was taken to the US without her parents and adopted.

Although reunited with her parents after the war, "it wasn't a fairytale ending. They were completely traumatised and my mother and I didn't even share a common language. I had forgotten my French."

All seven of her mother's siblings died in the Shoah, along with all Mrs Salter's cousins.

"Looking back, it's impossible to believe what happened - it's like looking through a cracked mirror."

Among the 550 guests were party leaders Jeremy Corbyn and Tim Farron and actors Dame Kristin Scott Thomas, Freddie Fox, Sheila Hancock, Kevin Whately and Robert Lindsay, who hosted the ceremony, The event was organised by the Holocaust Memorial Day Trust, whose chief executive Olivia Marks-Woldman reported that the 6,300 commemorations taking place across the country represented a massive increase on last year's total.

The HMDT also revealed that 700 more survivors have had their testimonies recorded over the past year.

"Holocaust Memorial Day makes a difference," Ms Marks-Woldman told the ceremony. People are still talking to their children about what they learned last January."

Chief Rabbi Ephraim Mirvis observed that "if only there would have been tens of thousands of people who stood up to their commanders to say: 'We refuse to obey your commands to murder people because they belong to a particular faith', millions of lives could have been saved.

"Millions of Jewish souls perished in a sea of silence. Let us declare: 'Never again'. No ifs, no buts."

Communities Secretary Greg Clark said: "Genocide is made not just by the choices of those who perpetrate it, but by those who let it happen.

"On behalf of the government, we commit never to look away, to challenge hatred whenever and wherever we see it."