The Queen's marriage to Prince Phillip is the longest of any British sovereign. And next month, the Royal couple will reach the milestone of 73 years of marriage. But for the past seven decades, the couple have reportedly managed to keep one secret closely guarded — an inscription inside their wedding ring.

The British Queen's wedding ring, made in Welsh gold, is inscribed with a secret message, that even those closest to the Nonarch don't know, and was chosen by then young Prince Philip in 1947, a new book has revealed.

"At least 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," royal biographer Ingrid Seward wrote.

According to the book, 'Prince Philip: A Portrait of the Duke of Edinburgh', the Queen's engagement ring was also a gift, as her young husband then made it from a tiara that belonged to his mother, Princess Alice of Battenberg.

Princess Alice had received it as a gift from Tsar Nicholas II of Russia when she married Prince Andrew of Greece and Denmark. And before proposing, Philip, now 99, had the tiara taken apart and repurposed the diamonds to make a ring for his bride-to-be, now 94,  the 'Daily Mail' newspaper reported.

The couple got married on November 20, 1947, at Westminster Abbey.