He was 11 years old when his mother Diana, Princess of Wales, took him to a homeless shelter for the first time.
That visit to The Passage made a huge impression on the young Prince William – one that has inspired him to teach his own children, Prince George, Princess Charlotte and Prince Louis, about the plight of those living on the streets or in temporary accommodation.
Now it is driving one of his most ambitious projects to date – a mission to prove that homelessness can be eradicated.
As he launches his five-year programme to tackle the problem across the UK in what would have been her birthday week, the Prince is honouring his late mother's memory.
A royal source tells HELLO!: “This is something that means so much to him, and it meant so much to his mother. It was that first visit to The Passage with his mother that has really inspired this work and he has been across every detail of this project.”
William will tour six locations across the four nations this week to unveil his Royal Foundation's new Homewards programme.
Each area will be supported by experts, partners and funders - including more than 15 homelessness organisations – to come up with a locally-led action plan. The Royal Foundation will also provide each location with up to £500,000 flexible seed funding to test new ways to deliver homes.
William wants to demonstrate that homelessness can be ended, making it rare, brief and unrepeated. And he has brought in celebrity advocates including DJ Sara Cox, TV presenter Gail Porter, England footballers Fara Williams and Tyrone Mings, opera star Sir Bryn Terfel and former Spice Girl Geri Horner to help raise awareness.
The Prince hopes the plan will create a blueprint for tackling the issue elsewhere in the UK and overseas.
He says: "In a modern and progressive society, everyone should have a safe and secure home, be treated with dignity and given the support they need.
"Through Homewards, I want to make this a reality and over the next five years, give people across the UK hope that homelessness can be prevented when we collaborate.
"I am fortunate to have seen first-hand the tireless work of people and organisations across the sector, the tangible impact their efforts can have and what can be done when communities are able to focus on preventing homelessness, rather than managing it.
"It's a big task, but I firmly believe that by working together it is possible to make homelessness rare, brief, and unrepeated and I am very much looking forward to working with our six locations to make our ambition a reality."
Understanding he is in a hugely privileged position has made the Prince determined to take action.
He has also been speaking to Prince George, Princess Charlotte and Prince Louis about the problem on their school run, following his late mother’s example.
"When we were in London, driving backwards and forwards, we regularly used to see people sitting outside supermarkets and we'd talk about it," he revealed in an interview with the Sunday Times.
"I'd say to the children, 'Why are they there? What's going on?' I think it’s in all our interests, it's the right thing to do, to expose the children, at the right stage, in the right dialogue, so they have an understanding. They [will] grow up knowing that actually, do you know what, some of us are very fortunate, some of us need a little bit of a helping hand, some of us need to do a bit more where we can to help others improve their lives."
He also plans to develop social housing on the 130,000-acres of land he has inherited through the Duchy of Cornwall, but is determined to find sustainable long-term solutions to the issue rather than delivering a quick PR stunt.
William's in-depth knowledge of the issues around homelessness comes from his work with charities such as Centrepoint, which became one of his first patronages in 2005. He took part in its 2009 Sleepout, became patron of The Passage in 2019 and last year sold the Big Issue on the streets of London.
"For him, this is something that up there with Earthshot as one of his most important projects," the royal source says of the Homewards programme. "This isn’t just five years, it’s the rest of his life. This isn't just his legacy. It's his mother's legacy and he is committed to this. He is committed to transforming the way that we as a nation think about homelessness."
The Prince says of those experiencing homelessness: "They've become invisible. It's really important that society acknowledges that there is somebody there and they're having a tough time. It shouldn't happen, but it's right there. You can't ignore it."
New research shared by The Royal Foundation suggests that one in five people in the UK have some experience of homelessness (either themselves or through someone close to them). More than 300,000 people in the UK - nearly half of whom are children – are currently sofa surfing, sleeping on the streets, living in cars, staying in hostels and other types of temporary accommodation.
According to Matt Downie, CEO of Crisis, the national homelessness charity, the problem is getting worse, due to a "severe shortage of genuinely affordable homes".
He says: "We're seeing thousands of households straining under the constant pressure of rising rents and the increasing cost of living. Councils in England spend 1.6 billion pounds a year on keeping people homeless in temporary accommodation. Preventing this by providing secure housing is not only better for individuals and families, it’s also better for the public purse."
A Housing First policy to end homelessness has already proved successful in Finland, with other countries adopting the same approach. The scheme sees homeless people offered accommodation first, then helped to find employment and tackle problems like addiction so they can start to pay rent.
While William is careful not to get involved with politics, he has spoken to the Levelling Up Secretary, the First Ministers of Scotland and Wales and the chief civil servants of Northern Ireland in recent weeks, to make them aware of his plan.
Homewards advocate Gail Porter, who has experienced homelessness says she is taking part "to raise awareness" and "take those feelings of embarrassment away and get people the help that's needed".
She tells HELLO!: "Ultimately, I hope that through Homewards someone as influential as Prince William is able to generate enough activity and focus to put an end to homelessness. Having spoken to him about this programme, I can see how much he wants to make a difference, as do I, so I hope that we can do that because things need to change."
Fara Williams, who has also experienced homelessness, agrees.
"For too long we've talked about homelessness as if it's an accepted part of life," she tells HELLO! "I hope that we can show that it doesn't need to be inevitable and inspire everyone that it can be ended."
Homewards looks set to be a defining project for the heir to the throne, who will be "rolling up his sleeves" to get things done, according to a royal source.
As Amanda Berry, chief executive of the Royal Foundation explains: "He's at the heart and soul of this."