In an ITV documentary earlier this year, William said "inspiration and guidance" from Diana had been a driving force behind his personal commitment to tackle homelessness.
He also recalled first visiting the passage with his mother, which included playing chess and chatting to people at the shelter.
"I must have been about 11, I think, probably, at the time. Maybe 10. I'd never been to anything like that before. And I was a bit anxious as to what to expect," he said.
"I remember at the time, kind of thinking, well, if everyone's not got a home, they're all going to be really sad.
"But it was incredible how happy an environment it was," he said.
"That's when it dawned on me that there are other people out there who don't have the same life as you do."
In the documentary, William admitted he sometimes felt guilty about not being able to do more - and wanted to share with his own children a sense of empathy for those facing hardship.
"When I was very small, my mother started talking about homelessness, much like I do now with my children on the school run," the prince said.
The prince has continued visiting the charity throughout his life and became its official patron in 2019.