The I’m A Celebrity... Get Me Out Of Here! presenter and his partner Declan Donnelly joined the Duchess of Cambridge at the launch of the Taking Action on Addiction campaign for The Forward Trust, which aims to improve the understanding of addiction.
Talking to Ant and Dec ahead of her speech, the Duchess said: “The more we speak to everybody, and actually some of the stories today, the feeling of a burden that we have to carry, (it’s like) a secret event.
“And actually once you start sharing your stories, there are so many people who have either experienced it themselves or know someone who has gone through it."
McPartlin, 45, replied: “I found that myself.
“By the time I asked for help it was really, it was bad, but as soon as you opened up to people, problems have got to disappear and it gets better and help is there.”
The Saturday Night Takeaway added that "Just asking for help", can be the first big hurdle.
The Duchess said: "That's what we're really passionate about – it's prevention. To get in there before it gets to crisis point."
Before their heart-to-heart chat Ant and Dec had shared a joke with the Duchess of Cambridge when she was taken by surprise to find them waiting to meet her.
She said: "I felt like I was walking into a Bushtucker trial!" to watch Donnelly replied; "We can arrange that!"
McPartlin pled guilty to drink-driving in April 2018 after he was found to be more than than twice the legal alcohol limit while driving his Mini in London in March 2018.
He was suspended from work at ITV for several months and sought rehabilitation treatment, leaving Donnelly to host Britain’s Got Talent and I’m A Celebrity without him.
The TV presenter revealed afterwards: "I saw Dec the day after the crash and we didn’t even speak work. It wasn’t about that. It was about, ‘How are you?’ I said: ‘I’m not right.’ You know, mentally I wasn’t in a good place."
Watch: Kate Middleton warned in her speech 'no one is immune to addiction'