Belsen survivor Rudi Oppenheimer told London Mayor Boris Johnson and other civic leaders at the HMD ceremony at City Hall: “Soon there will be no one left to tell the story.”

Mr Oppenheimer estimated that he had related his experiences to 90,000 young people in 950 sessions at schools. “I am the witness,” he said.

The mayor declared that as no words of his could be as powerful as those of a survior, he had chosen to read If, a “shocking and frightening poem” by playwright Edward Bond which pondered the outcome “if Auschwitz had been in Hampshire”.

London students who had participated in the Holocaust Educational Trust’s Lessons from Auschwitz Project recalled their visits to the death camp. For Zoe Le Coultre, from Brentford School for Girls, the most upsetting aspect had been seeing a pair of red high-heel shoes among a pile of footwear that would have belonged to a teenager like herself. “Each pair of shoes told its own story.”

Anne Novis of the UK Disabled People’s Council spoke about discrimination against the disabled and Mr Oppenheimer and Lessons from Auschwitz graduate Chloe Kimmel lit a memorial candle.

The ceremony was organised by the London Assembly, the HET and the HMD Trust.

An exhibition, Portraits for Posterity, about survivors living in Britain, lined the walls of the corridor leading to the auditorium.

Speaking afterwards, Mr Johnson said he had been to concentration camps in Lithuania and hoped to visit Auschwitz on an HET trip in the near future.