The Duke of Cambridge listened in to a call from a desperate homeless teenager as he launched Centrepoint's new helpline for young rough sleepers needing urgent advice.
The young man, who had been robbed twice while sleeping rough in London, had rung the new number after failing to get help from his local authority and a concerned William asked to be kept informed about his progress.
Centrepoint hopes its new helpline will assist thousands of young people who are in need of help, support and advice after finding themselves without a permanent home or at risk of becoming homeless.
The Duke joined Cerys Lewis, 36, a helpline advice worker, at a central London office as she took one of the first calls on the helpline which has been running as a pilot programme since last April.
During the 20-minute call, Ms Lewis talked through the available options with the teenager, who had been sleeping rough and bedding down in all-night cafes following a breakdown in his relationship with his parents.
After recommending a night shelter, she told him: "It's not ideal, but it's safe, it's free and you'll get your evening meals and breakfast in the morning."
At the end of the call, William, who is patron of Centrepoint and had been listening on another phone, said to her: "Will you let me know how he gets on? I'd like to know."
Before the Duke listened in to the call, he chatted with Ms Lewis and when she mentioned the catchment area of the helpline, William replied: "It will be really interesting to work out in the country where the hotspots of homelessness are."
The Centrepoint helpline, run in partnership with the youth support charity The Mix, will initially be open from 9am-5pm, Monday to Friday, covering England, and will expand if demand increases.
William was joined at its launch by Centrepoint chief executive Seyi Obakin and BBC Radio 2 DJ and Centrepoint ambassador Sara Cox.
Mr Obakin said: "It's absolutely fantastic to see the helpline up and running and reaching out to young people.
"Today, I have to tell you, is an important day, not just for those who are homeless. Do you know why? Because homelessness is not that far away and it can happen to anyone, and what the helpline is going to do is give them a lifeline."
Centrepoint commissioned an online YouGov poll on the issue which questioned 2,004 16 to 25-year-olds last September.
The poll found that 17% of those quizzed said at some point they felt they had nowhere safe to call home. As a result of this, 65% had sofa-surfed, 35% had remained where they were living even though it was not safe, and 31% had slept in an insecure place, such as on the streets, in a car or tent, or squatted.
Speaking about William's presence at the launch, the chief executive added: "It's a signal of how important he himself sees the helpline and how much difference he thinks the helpline will make to young people, that's why he personally wanted to be here to launch it."
Earlier, William joined a training session for two new helpline advice workers and sat with Evgeny Lebedev, proprietor of the Independent and Evening Standard newspapers which made the Centrepoint helpline their Christmas appeal, along with the i newspaper, raising more £3 million.
The Duke took on the role of a helpline adviser while Samia Meah, 27, who is undergoing training, played a young person who had been served an eviction notice by their landlord after falling behind with the rent.
Watching was Cox, a Centrepoint ambassador for 15 years, who said of youth homelessness: "I just feel for this young group - 18 to 25-year-olds - they have so much potential, just on the cusp of life and, through no fault of their own, find themselves in a difficult position."
She added that Centrepoint was not just a "bed for the night" but helped with education and employment.