Britain's Queen Elizabeth II and Prince Philip are celebrating their 73rd wedding anniversary today.

For the occasion, the royal couple have released a photograph of themselves opening a card from three of their great-grandchildren.

The photo made public on Friday shows Britain's longest-reigning monarch and her husband seated on a sofa at Windsor Castle while examining the card made by the children of their grandson Prince William and his wife, the Duchess of Cambridge.

The card from George, Charlotte and Louis featured a pop-up number 73.

Elizabeth was only 21 when she married Royal Navy Lieutenant Philip Mountbatten at Westminster Abbey on November 20, 1947. She has been queen since 1952, ruling longer than any other British monarch.

Her marriage to Prince Philip is also the longest for any British sovereign.

The couple first met at the wedding of Princess Marina of Greece and Denmark to Prince George, Duke of Kent, in 1934. Some time later they started writing letters to each other. The couple became engaged in 1946 without asking permission from her parents, controversially, but not long after Philip asked George VI for his daughter's hand in marriage.

The royals married on November 20, 1947, at Westminster Abbey, with more than 200 million people watching the service on TV from around the world and 2000 guests attending.

While Queen Elizabeth continues to carry out her duties, Prince Philip, 99, has retired from public life. With England under a national lockdown to control the Covid-19 pandemic, the couple is self-isolating at Windsor Castle.

In the seven decades since they got married, the couple have kept one closely guarded secret.

Just like Meghan's and Kate's, the Queen's wedding ring is made from Welsh gold.

What sets it apart is that it's engraved with a secret message chosen by Philip in 1947 - and even those closest to the monarch don't know what it says.

Royal author Ingrid Seward writes in her new book Prince Philip: A Portrait of the Duke of Edinburgh that "Philip didn't have the expense of a wedding ring, as the people of Wales supplied a nugget of Welsh gold from which the ring was made".

"She never takes it off and inside the ring is an inscription.

"No one knows what it says, other than the engraver, the Queen and her husband."

Philip also had her engagement ring made from a tiara that belonged to his mother, Princess Alice of Battenberg.

Before he proposed, Philip had the tiara taken apart and the diamonds repurposed to make a ring for his bride-to-be.

He then designed the three-carat solitaire with five smaller diamonds.

- Additional reporting, NZ Herald