It's World Book Day 2021 and the Royal Family has marked the occasion with a throwback picture of Queen Elizabeth II and Prince Philip. The picture may come as a tribute to the Duke of Edinburgh, who is currently recovering in hospital after undergoing a "successful" procedure on his heart on Wednesday.

Queen Elizabeth & Prince Philip's World Book Day 2021 photo

On Instagram, the Royal Family led off a three-part post for World Book Day with a 1976 photograph of Queen Elizabeth and the Duke of Edinburgh at the library in Balmoral Castle. Over four decades ago, the Queen is seated while the Duke, standing, supports her with an arm from behind, surrounded by full shelves of books.

Also included in the post are a painting of Queen Victoria and Prince Albert in the Royal Library at Windsor Castle, and a video of Duchess Camilla discussing the joys of reading and owning your first book.

Royal fans wish Prince Philip well

The Royal Family's post was flooded with comments from well-wishers, taking the opportunity to send their best to Prince Philip. "Wishing a speedy recovery for Prince Philip," reads the top-voted comment, followed by many similar messages.

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Prince Philip underwent a heart procedure at St. Bartholomew's Hospital this week.

The 99-year-old Duke has now been in hospital for 16 days. He was initially being treated for an infection but was relocated to St. Bartholomew's Hospital earlier this week. Today, Buckingham Palace announced that Philip underwent a "successful procedure for a pre-existing heart condition" on Wednesday and that he will "remain in hospital for treatment, rest and recuperation for a number of days."

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