Prince Charles' last conversation with his late father, Prince Philip, revealed in new documentary

Some of the last words Prince Charles exchanged with his late father, Prince Philip, have been revealed.

In an upcoming documentary by BBC One about the legacy left by Queen Elizabeth's husband of 73 years, Prince Charles reportedly reveals he spoke on the phone with the Prince of Edinburgh one day before his death.

Philip died at the age of 99 on April 9, nearly two months shy of his 100th birthday. The DailyMail reports that Charles and Philip were discussing how to celebrate the big milestone in what is believed to be their final conversation.

"We're talking about your birthday," Charles said to his father in a phone call on April 8, according to the outlet. 

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Charles allegedly repeated again, "We're talking about your birthday! And whether there's going to be reception!"

Philip, who was at his home in Windsor following a hospitalization, cracked, "Well, I've got to be alive for it, haven't I?"

"I knew you'd say that!" Charles replied, via the outlet.

BBC announced earlier this month that the upcoming documentary "Prince Philip: The Royal Family Remembers" will feature all four of Prince Philip and the queen's children: Charles, 72, Princess Anne, 71, Prince Andrew, 61, and Prince Edward, 57. 

The project was originally produced to mark Philip's 100th birthday on June 10.

"There are poignant recollections, plenty of humour and numerous fresh insights into the character and legacy of this royal pioneer. With special access to the Queen’s private cine-film collection, this film is an unrivalled portrait of a man with a unique place in royal history - by those who knew him best," the BBC said in a statement.

Prince Charles offered a heartfelt tribute to his "dear Papa,'' in April following his death.

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"I particularly wanted to say that my father, for I suppose the last 70 years, has given the most remarkable devoted service to The Queen, to my family, and to the country, but also to the whole of the Commonwealth. And as you can imagine, my family and I miss my father enormously. He was a much loved and appreciated figure and apart from anything else I can imagine he would be so deeply touched by the number of other people here and elsewhere around the world and the Commonwealth who also I think, share our loss and our sorrow," Charles said, speaking from his southwestern England home of Highgrove.

"My dear Papa was a very special person who I think above all else would have been amazed by the reaction and the touching things that have been said about him,″ he continued. "And from that point of view we are, my family, deeply grateful for all that. It will sustain us in this particular loss and at this particularly sad time.’’

The royal family confirmed the Duke of Edinburgh's death on April 9 in a statement.

"It is with deep sorrow that Her Majesty The Queen has announced the death of her beloved husband, His Royal Highness The Prince Philip, Duke of Edinburgh. His Royal Highness passed away peacefully this morning at Windsor Castle," the statement said.

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Philip was hospitalized months before his death. On Feb. 16, the prince was admitted to a London hospital after feeling unwell. On March 3, he underwent a procedure for a pre-existing heart condition at St. Bartholomew’s Hospital before being transferred back to King Edward VII hospital on March 5 and ultimately released home on March 16.

Philip married then-Princess Elizabeth in 1947 and is the longest-serving royal consort in British history.