The royals, PM Boris Johnson and faith leaders attended the service in London, to mark 75 years since the former Nazi death camp of Auschwitz was liberated.

The duke gave a reading and the couple spoke to survivors of the Holocaust and more recent genocides.

Earlier, dozens of world leaders joined survivors at Auschwitz in Poland.

At Central Hall in Westminster, survivors of the Holocaust and other genocides, and their relatives, spoke of their experiences during an hour-long event hosted by BBC newsreader Huw Edwards.

Holocaust survivors Arek Hersh, Mala Tribich, Yvonne Bernstein, Eva Clarke, Rachel Levy and Manfred Goldberg all lit ceremonial candles.

And there were dramatic readings of first-hand accounts of the horrors experienced during the genocide by actors Nina Wadia, Rebecca Front, Martin Shaw and Sir Simon Russell Beale.

In a speech, Prince William paid tribute to his great-grandmother Princess Alice, who he said risked her own life to save a Jewish family - the Cohens - in Athens in 1943.

Alice's bravery was recognised by Israel which in 1993 posthumously bestowed the title of Righteous Among the Nations on her.

Reading from a letter written by a friend of the princess, Prince William said: "The members of the Cohen family left the residence three weeks after liberation, aware that... the princess's generosity and bravery had spared them from the Nazis."

The Duchess of Cambridge later praised survivors and their relatives for sharing their "heartbreaking" stories at the event and she said she and William found the ceremony "very poignant".

Mr Johnson told those listening he felt a "deep sense of shame" that anti-Semitism continued in the UK today.

The PM said Britain seemed "to be dealing with a resurgence of the virus of anti-Semitism", saying: "I know that I carry a responsibility as prime minister to do everything possible to stamp it out."

He vowed to ensure the horrors of the Holocaust were not forgotten and lent his support to the proposed National Holocaust Memorial and Education Centre near Parliament.

"As prime minister I promise that we will preserve this truth forever," he said.

Among those at the ceremony were the Archbishop of Canterbury Justin Welby and the Chief Rabbi, Ephraim Mirvis.

The UK commemoration in Westminster honoured survivors of the Holocaust - also known as the Shoah, in which millions of Jewish people were killed - Nazi persecution, and the genocides which followed in Cambodia, Rwanda, Bosnia and Darfur, according to organisers.

Around one million people - many of them Jewish - were killed at Auschwitz in Nazi-occupied southern Poland before it was liberated by the Russian army on 27 January 1945.

Some 200 Holocaust survivors - including some who are now living in the UK - returned to the former Nazi death camp at Auschwitz for a commemoration.

Batsheva Dagan, who was given the number 45054 on arrival at Auschwitz, told those gathered in Poland that "human dignity did not belong" at the camp.

"Quite the opposite," she said. "Human dignity was trampled."

Earlier on Monday, two portraits of Holocaust survivors taken by the Duchess of Cambridge for a forthcoming exhibition were released to mark the anniversary of the liberation of Auschwitz.

The UK Holocaust Memorial Day commemorative event aired on BBC Two at 1900 GMT and will be available on the BBC iPlayer. soon.