Recalling a fundraising gala in November 2013 for the homeless charity Centrepoint, a cause first championed by his late mother Princess Diana, the 39-year-old chuckles and says: ‘I can’t believe I’m actually telling you this story… I turn up and Jon Bon Jovi and Taylor Swift are at the event, which nearly knocked me off my feet.
‘And then, I went inside, met everybody, a lot of handshaking to do those nights, a lot of chatting. I try and be charming and interactive.
‘Occasionally, you’re not having a good day, and it’s a little harder to come by.
‘When I sat down to watch Jon Bon Jovi do his performance, I thought, “That’s it. My job is done. I’ll get a dinner in a minute, and I might be able to have a chat to some people, and, you know, I’m off-duty a little bit now.” ‘
But matters did not go according to that plan.
‘Little did I think what was going to happen next,’ he adds. ‘I’m sat next to Taylor Swift.
‘She’s on my left – and after Jon does his first song, there’s a pause and she turns to me.
Taylor Swift, Jon Bon Jovi and Prince William, Duke of Cambridge sing on stage at the Centrepoint Gala Dinner at Kensington Palace on November 26, 2013 in London
‘She puts her hand on my arm, looks me in the eye and says, “Come on, William. Let’s go and sing.”
‘To this day, I still do not know what came over me. Honestly, even now I’m cringing at what happened next, and I don’t understand why I gave in.
‘But, frankly, if Taylor Swift looks you in the eye, touches your arm and says, “Come with me…” I got up like a puppy and went, “Yeah, OK, that seems like a great idea. I’ll follow you.”
‘I walked up on stage in a trance and then sort of halfway through Jon Bon Jovi’s Livin’ On A Prayer song, I wake up.
‘And I’m thinking to myself, “Am I standing on the stage singing Livin’ On A Prayer when I don’t even know the words?”
‘But the Centrepoint young guys and girls were there, all loving it and cheering away.
‘So I thought, “Well, if they’re enjoying it, then the night is for them. So sod it. I can’t be the doofus who’s going to ruin it for everyone.”
‘And so I desperately try and remember some of the words and sing as hard as I can.
‘Beneath my black tie, there was a lot of sweating going on. I felt like a swan, where I was trying to keep myself composed on the outside, but inside, the little legs are paddling fast.
‘Now, a lot of people might think I’m comfortable on stage. When I do speeches and things like that, I’ve done so many now, they’re not a problem. But I’ve not done singing.’
William’s appearance on stage –just four months after he had become a father for the first time – drew comparisons to the event in 1978 when Prince Charles celebrated his 30th birthday by dancing on stage with The Three Degrees.
STAR QUALITY: William and Miss Swift after their number at the Centrepoint gala in 2013
But it quickly becomes clear that, far from parading his association with the world of celebrity, William has a more serious point.
‘At times, when you’re taken out of your comfort zone, you’ve got to roll with it,’ he says.
‘And I think we’ve got to the stage in this life where we do micromanage ourselves.
‘We do worry about: how do we look on social media?
‘Who said what about me? What am I wearing? There’s so many pressures, but I think that making a fool of yourself is OK.
‘It’s OK to not take yourself too seriously and have those moments where you let go and you just go, “Do you know what? I’m OK with this.” ‘
While William, who has been Patron of Centrepoint since 2005, may wince at the memory of his performance, Ms Swift, 31, who has sold more than 200 million records worldwide and who is dating English actor Joe Alwyn, has been complimentary.
In an interview with The Mail on Sunday shortly after the event, she said: ‘I could hear his voice and it’s pretty good. A nice tone. I’d definitely say he has got star quality and if he ever wants me to sing with him again, I’ll be there.’