Prince William opened up to a girl who was grieving for her father in a bid to comfort her, saying: "I lost my mummy when I was very young too."
The prince, accompanied by his wife Kate Middleton, was visiting the Child Bereavement UK Centre in Stratford which provides support for children coming to terms with the loss of a loved one.
Speaking with children and their parents, the father-of-two met nine-year-old Aoife and her brother Keenan, 12, whose father John died from pancreatic cancer six years ago.
Sharing his own experience of loss as a child the prince — who will mark the 20th anniversary of the death of his mother Diana, Princess of Wales, later this year — told Aoife: "Do you know what happened to me? You know I lost my mummy when I was very young too. I was 15 and my brother was 12."
Encouraging her to share her feelings about the loss of her father, the 36-year-old asked: "Do you speak about your daddy? It's very important to talk about it. Very, very important."
Turning to Aoife's mother he added: "I think that for children it is sometimes difficult to understand that. "
Aoife's mother, Marie, said that the prince's willingness to open up about his own grief to help her daughter, moved her to tears. "I couldn't believe it when he started to talk about his mother. It was very emotional and I was willing myself not to start to cry. I almost did."
"I am telling my children that if they take anything away from this day, it is what he said about how important it is to talk. Kids do not forget that. Sometimes it hurts but we can remember the happy things too. It is so important to talk."
Earlier William and Kate joined the children in making "memory jars", which were filled with layers of multi-coloured salts, each colour representing a memory of their loved one.
Describing the sudden death of her husband and the impact of his loss on her children Marie, from Redbridge, said: "Pancreatic cancer is very aggressive and difficult to diagnose and John didn't fit the bill at all — he was young, healthy and ate well. We lost him very quickly after he was finally diagnosed. My children were almost six and three. My son wanted to talk about it but my daughter was finding it very hard and didn't want to.
"It took me a long time to find Child Bereavement UK, we have been working with them since June, but what they have done for us as a family has been remarkable. Aoife didn't want to sit down with a counsellor one-on-one, but they work there with activities, enabling her to discuss her feelings in a more relaxed way."
Speaking about Prince William, Aoife said: "It was really nice that he talked to me. It was like there are other people who know what it is like to lose someone."
Prince William has been the Royal Patron of Child Bereavement UK since 2009, and has supported the work undertaken by the charity which, as well as helping people cope with loss, provides training for 8,000 professionals a year who work with bereaved families.
Child Bereavement UK was established in 1994 in the presence of Diana, Princess of Wales, whose best friend, Julia Samuel, is founder patron of the organisation. Samuel has remained close to William and was asked to be Prince George's godmother according to Mail Online.
Speaking after the event, Samuel praised the prince for his warmth and understanding.
"Bereavement – and particularly child death which is so unbearable to think about – is something he's shining a light on and helping us raise awareness of the need of people who have children who die in the way that no one else can do. That's because people can see how authentic, and heartfelt, he is and he's experienced it. People can pick up authenticity within seconds."
"Him just coming here because of his position raises awareness. We were going for 10 years before he became our patron and we did well. But the 10 years since he's become patron have been transitional. William has great depth of understanding in a way that you never expect from such a public figure and yet he is also so friendly and funny," she said.
"You can see when people meet him that their eyes widen a little when they recognise him but when he talks to them about what has happened to them and himself, then he very quickly becomes a human being. I don't think any of us can even begin to understand what he and Harry went through. Their mother's death was such a public event and yet they were feeling such a depth of personal grief.
"What they have both learnt and are so keen to get across, however, is that talking about your grief at the earliest possible stage is so important. The earlier we talk about these things, the less harm is done.
"What William is bringing to our charity is just immeasurable. He has lived and breathed what these families are experiencing. He is really involved as a patron and believes in the service we can offer. Grief is an invisible wound, people don't know what you are suffering, they don't offer you a chair to sit on because you look ill. But William knows and recognises this and I think people respond to this."
The prince, who lost his mother in a car crash in 1997, aged just 36, has previously opened up about his grief as he struggled to cope with her loss.
"Never being able to say the word 'mummy' again in your life sounds like a small thing," he said. "Life is altered as you know it, and not a day goes past without you thinking about the one you have lost.
"'I know that over time it is possible to learn to live with what has happened and, with the passing of years, to retain or rediscover cherished memories."