She wrote: “Shown to the green retiring room behind the [royal] box, I collapsed onto a sofa and was offered a cup of tea. Suddenly, a vision appeared in a red dress. Catherine, the [then] Duchess of Cambridge, with her two young children in tow.

“‘Hello, Joan,’ she said sweetly, holding out her hand.

“‘Hello, ma’am,’ I gulped, forgetting to curtsy.

“‘This is Charlotte, and this is Louis,’ she said, and both little people held out their tiny hands politely for a handshake.”

There are no obligatory codes of behaviour when meeting The Queen or a member of the Royal Family, but many people wish to observe the traditional forms, the official royal website states.

Men are encouraged to greet royals with a neck bow while women can curtsy but a simple handshake in the usual way is also an acceptable greeting.

Royal fans may recall Louis taking the limelight at the Platinum Jubilee celebrations as he pulled cheeky faces during the ceremony.

Prince William, Kate and their three children, Prince George, Charlotte and Louis watched the pageant from the royal box along with extended members of the late Queen’s family.