Prince Philip's cause of death has been revealed.

The Duke of Edinburgh, who died on April 9 at the age of 99, died of "old age," according to a death certificate obtained by The Telegraph.

Sir Huw Thomas, who is the head of the royal medical household, certified Philip's death certificate. Philip's cause of death was declared as "old age" and is an accepted description for a cause of death if the patient is over 80 years old and whose health has been recognized as on a "gradual decline" after having been cared for over a long stretch of time, the outlet reports.

Additionally, Sir Thomas' analysis proves there was no other disease or injury that contributed to Philip's death. This means the heart condition that caused the duke to undergo surgery in March did not contribute to his death, the outlet states.

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In this undated handout photo provided by the Duke and Duchess of Cambridge, Britain's Queen Elizabeth and Prince Philip with their great grandchildren, from left, Prince George, Prince Louis, Savannah Phillips (standing at rear), Princess Charlotte, Isla Phillips holding Lena Tindall, and Mia Tindall. Britain's Prince Philip, the irascible and tough-minded husband of Queen Elizabeth II who spent more than seven decades supporting his wife in a role that mostly defined his life, died on Friday, April 9, 2021.

In this undated handout photo provided by the Duke and Duchess of Cambridge, Britain's Queen Elizabeth and Prince Philip with their great grandchildren, from left, Prince George, Prince Louis, Savannah Phillips (standing at rear), Princess Charlotte, Isla Phillips holding Lena Tindall, and Mia Tindall. Britain's Prince Philip, the irascible and tough-minded husband of Queen Elizabeth II who spent more than seven decades supporting his wife in a role that mostly defined his life, died on Friday, April 9, 2021. (Duchess of Cambridge via AP)

His private secretary, Brigadier Archie Miller-Bakewell, registered Philip's death with the Royal Borough of Windsor and Maidenhead on April 13. According to the Telegraph, his full name on his death certificate is listed as "His Royal Highness The Prince Philip, Duke of Edinburgh formerly known as Prince Philippos of Greece and Denmark formerly known as Philip Mountbatten."

Buckingham Palace representatives declined comment.

The royal family confirmed the death of Queen Elizabeth II’s husband of 73 years 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 read.

On his death certificate, Philip's occupations are listed as a naval officer and the "husband of Her Majesty Queen Elizabeth II, The Sovereign."

Philip married then-Princess Elizabeth in 1947 and is the longest-serving royal consort in British history. He and the queen have four children, eight grandchildren and 10 great-grandchildren. 

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The Duke of Edinburgh, <a href="https://foxnews.1eye.us/category/world/personalities/queen" target="_blank">Queen Elizabeth II</a>’s husband of 73 years, <a href="https://foxnews.1eye.us/category/entertainment/events/departed" target="_blank">died</a> on April 9. Here, the queen and Prince Philip chatting during the royal windsor horse show on the grounds of Windsor Castle years ago.

The Duke of Edinburgh, Queen Elizabeth II’s husband of 73 years, died on April 9. Here, the queen and Prince Philip chatting during the royal windsor horse show on the grounds of Windsor Castle years ago. (im Graham Photo Library via Getty Images)

During England’s coronavirus lockdown, he had been staying at Windsor Castle, west of London, with the queen, 95.

On Feb. 16, Philip 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.

In 2011, he was rushed to a hospital by helicopter after suffering chest pains and treated for a blocked coronary artery. In 2017, he spent two nights in the King Edward Hospital and was hospitalized for 10 days in 2018 for a hip replacement. 

Philip was also hospitalized in December 2019, spending four nights in the King Edward Hospital for what the palace said was planned treatment of a pre-existing condition. 

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He was forced to give up driving at the age of 97, after smashing into a car while driving a Land Rover near Sandringham estate in January 2019. Philip needed help to get out of the Land Rover but wasn’t injured. A woman in the other vehicle suffered a broken wrist. 

Princess Elizabeth with her husband Prince Philip, Duke of Edinburgh, and their children Prince Charles and Princess Anne, August 1951.

Princess Elizabeth with her husband Prince Philip, Duke of Edinburgh, and their children Prince Charles and Princess Anne, August 1951. (Keystone/Hulton Archive/Getty Images)

In a rare interview for a television documentary to mark his 90th birthday in June 2011, he said, "I reckon I've done my bit. I want to enjoy myself for a bit now. With less responsibility, less rushing about, less preparation, less trying to think of something to say. 

"On top of that, your memory's going. I can't remember names. Yes, I'm just sort of winding down." 

At the time of his death, Philip's full title was His Royal Highness Prince Philip, Duke of Edinburgh, Earl of Merioneth and Baron Greenwich, Knight of the Garter, Knight of the Thistle, Order of Merit, Knight Grand Cross of the Order of the British Empire, Companion of the Order of Australia, Companion of the Queen's Service Order, Privy Counselor. 

He was the second person ever to bear the title "Duke of Edinburgh," the first being his great-great-uncle, Prince Alfred Ernest Albert. His son, Prince Edward, will now assume the title. 

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A slimmed-down royal ceremonial funeral was held for the Duke of Edinburgh on April 17 in St. George's Chapel at Windsor Castle.

A minute of silence is observed at St George's Chapel for the funeral of Britain's Prince Philip inside Windsor Castle in Windsor, England, Saturday, April 17, 2021. 

A minute of silence is observed at St George's Chapel for the funeral of Britain's Prince Philip inside Windsor Castle in Windsor, England, Saturday, April 17, 2021.  (Kirsty Wigglesworth/Pool via AP)

Philip's celebration of life, which he had a major role in planning prior to his death, was attended by his and the queen's four children -- Prince Charles, Princess Anne, Prince Andrew and Prince Edward -- as well as Charles's wife Camilla, Duchess of Cornwall; Anne's husband Timothy Laurence and Edward's wife Sophie, Countess of Wessex.

Philip’s grandchildren -- which include Peter Phillips, Zara Tindall, Prince William, Prince Harry, Princess Beatrice, Princess Eugenie, Lady Louise and James, Viscount Severn — also were in attendance.

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Meanwhile, Prince William's wife, Kate Middleton, Zara Tindall's husband, Mike Tindall, Princess Eugenie's husband, Jack Brooksbank and Princess Beatrice's husband, Edoardo Mapelli Mozzi, will also join their royal spouses at the family event.

The Duke of Edinburgh served in the Royal Navy for 12 years. Military personnel had a large role in honoring him despite the attendance limit due to coronavirus restrictions in England.