Kate and William visited the former Stutthof camp in northern Poland where 65,000 people died during Germany's occupation in the Second World War from disease, malnutrition, physical exhaustion, exposure to the harsh climate and abuse from guards - as well as in the gas chamber used to murder those too sick to work, opposite a brick crematorium.
Both bowed their heads as they were shown inside the room today, which was used to burn the bodies of thousands of victims and Prince William - who was visibly moved - was overheard apologising to the museum director for asking 'so many questions' as they exited they crematorium.
Surrounded by a wire fence and watchtowers, the stark wooden barrack blocks that housed inmates in cramped conditions still stand while personal possessions, from combs and children's dolls have been turned into exhibits in memory of those who perished at the camp almost seven decades ago, alongside portraits drawn by artists incarcerated during the Holocaust.
The royals were shown discarded shoes and clothing seized from prisoners on arrival at the camp and the impact of the visit was clear from the heartfelt message the couple left in the visitors' book before leaving.
'This shattering visit has reminded us of the horrendous murder of six million Jews, drawn from across the whole of Europe, who died in the abominable Holocaust,' they wrote. 'All of us have an overwhelming responsibility to make sure that we learn the lessons and that the horror of what happened is never forgotten and never repeated.'
After a guided tour of the camp, 20 miles from Gdansk, they had an emotional meeting with two British survivors of Stuttoff, Zigi Shipper and Manfred Goldberg, both 87, who were both making their first return to the camp since moving to Britain after the war.
Mr Shipper said William and Kate - who declared they were 'honoured' to have met the pair - were clearly 'very moved' by what they saw, adding: 'You could see their faces. They were in pain.'
The friends were liberated in May 1945 by British tanks surrounding the barge on which he was about to be loaded. It was set to be towed out to sea and then blown up with Jews packed into it.