Prince Charles and his sons Prince William and Prince Harry recalled fond memories of the late Prince Philip in a forthcoming BBC documentary "Prince Philip: The Royal Family Remembers."

The Prince of Wales, the eldest child of the late Prince Consort, revealed the contents of the last conversation he had with his father just days before his death on April 9 this year. Charles recalled that he visited his father at Windsor Castle where he was recovering after his hospital stay, and delicately brought up the subject of his upcoming 100th birthday, reports Mail Online.

"We're talking about your birthday," Charles told his 99-year-old father, who according to various reports did not want any fuss on what could've been his milestone centenary birthday on June 10. The elderly royal had grown slightly hard of hearing at the time, so his son repeated himself again at a louder volume, "We're talking about your birthday and whether there's going to be reception!"

Prince Philip, known for his dry sense of humour, was clearly aware of what was awaiting him in the next few days. He told Charles, "Well, I've got to be alive for it, haven't I?"

Charles recalled in the documentary that he responded to his father, "I knew you'd say that!"

The recently-released trailer of the documentary also showed Princes William and Harry fondly remembering their grandfather and his witty humour. William recounted an incident where a young boy swore at Prince Philip while completing a hike for his Duke of Edinburgh Award, but received an unexpected reaction to his insult.

The Duke of Cambridge recalled, "[Philip] stopped and wound down his window and said, 'Good morning. How are you getting on?' To which the smallest young chap at the back turned round and effectively said, 'Jog on Grandpa!'"

William said that the boy's reply was actually much more blunt in reality, but his grandfather took it sportingly and found it hilarious. Philip wound the car window up before turning to William and saying, "The youth of today!"

Harry also recalled an example of his grandfather's "unapologetic" nature, and revealed the message the royal gave him when he was leaving to serve in war-torn Afghanistan.

"Going off to Afghanistan he was very matter of fact and just said, 'Make sure you come back alive'. Then, when I came back, there wasn't a deep level of discussion, more a case of, 'Well, you made it. How was it?' [and] that's how he was," the Duke of Sussex said.

Prince Philip Duke of Edinburgh
31 October 2015: Prince Harry, Prince William and the Duke of Edinburgh enjoy the-build up to the 2015 Rugby World Cup Final match between New Zealand and Australia at Twickenham Stadium Phil Walter/Getty Images