The Duke of Cambridge has urged fathers to discuss mental well-being with their children and families and be more open about their own feelings as he celebrated his third Father's Day as a parent.
William spoke about the importance of families discussing young people's mental health issues, warning that left unresolved they can "alter the course of a child's life forever".
And he encouraged fathers to overcome the common hurdle of struggling to talk about their own feelings, and not to neglect the often tricky topic of their children's mental well-being.
In an article published in The Sunday Express, William - father to George, two, and Charlotte, one - spoke of the importance of broaching mental health issues.
He said: "Today I celebrate my third Father's Day as a father. For me it is a day not just to celebrate how fortunate I am for my young family, but to reflect on just how much I've learned about fatherhood and the issues facing fathers in all walks of life.
"In particular, it is a time to reflect on my responsibility to look after not just the physical health of my two children, but to treat their mental needs as just as important a priority."
Highlighting how unresolved challenges from childhood can lead to issues such as addiction, suicide and homelessness in adolescence and adulthood, the Duke said: "While the circumstances of any one situation are unique, it is clear that many families could have been helped if they had found it easier to talk openly about mental health challenges in the home."
He added: "It is often said that fathers can often find it hard to talk about their own feelings so there's no wonder they struggle to speak to their son or daughter about the topic.
"But we don't really have a choice. I really believe that a child's mental health is just as important as his or her physical health."
William said a "generational shift" had taken place in attitudes to mental health, allowing a better understanding of things that in the past went unacknowledged.
He said: "A fifth of children will have a mental health issue by their 11th birthday. And left unresolved, those mental health issues can alter the course of a child's life forever.
"So on this Father's Day, I encourage all fathers to take a moment to ask their children how they are doing. Take the opportunity to discuss how you are coping with life and fatherhood with your wife, partner or with your friends.
"And know that if your son or daughter ever needs help, they need their father's guidance and support just as much as they need their mother's."