The world may yet feel "shame" at not taking further action in Syria, David Cameron warned as he launched a bid to ensure a "permanent and fitting" British memorial to the Nazi Holocaust.
The Prime Minister defended his support for military action against Bashar Assad's regime in a speech at a dinner to mark the 25th anniversary of the Holocaust Educational Trust.
His comments came as he described a report by UN weapons inspectors into the atrocity in Damascus as "chilling", adding on Twitter: "No-one can ignore facts. 100s gassed in worst attack since Halabja. UNSG right: we must destroy chemical weapons."
William Hague said the UN report was "damning" and "fully consistent" with Britain's assessment that Bashar Assad's regime was behind the attack. The Foreign Secretary spoke out after UN Secretary General Ban Ki-moon presented what he said was "overwhelming and indisputable" proof that chemical weapons were used on a large scale.
In his speech, Mr Cameron said society would not want to look back and wonder why more was not done, as with the genocides in Srebrenica and Rwanda. He told the dinner that one of the main lessons to learn from the Holocaust was "the lesson of not standing by", adding "with me as Prime Minister, Britain will never stand by".
"It's an extraordinary human emotion but somehow when genocide is taking place the shame of not acting sometimes doesn't quite register properly until afterwards," he said.
"When we look back at Srebrenica and Rwanda, we wonder now why we didn't do more at the time. When something truly terrible happens, it's as if we put up a defence mechanism and try and rationalise why we are powerless to act. The same could so easily be said of Syria."
He recounted his experience of watching videos of the aftermath of the chemical weapons attack in Damascus while he was on holiday in Cornwall, and was compelled to act.
Pointing out that the regime's stockpiles posed a "massive potential threat" to Israel, he said: "So there I was facing this reality. The evidence before our eyes. The flagrant breach of an international taboo against the use of chemical weapons. The refugee crisis of our time - in which nearly 500 will flee during this dinner tonight."
He added: What was my instinct? It wasn't to say what's the best way to handle this politically, to secure any advantage. It was to say what is the best way for my country to stand up. Because Britain is not the sort of country that wants to stand by."