• A 103-year-old letter written by Edward, the Prince of Wales to his married mistress Freda Dudley Ward in 1919 is up for auction
  • Edward recalled how he lured Lord Loughborough away so his younger brother Albert could be alone with Loughborough's wife
  • A books and manuscripts specialist described the letter as "unusual"

Queen Elizabeth II's father, King George VI, had an affair with a married woman in the years before he met and married his daughter's mother, according to a 103-year-old letter that's now up for auction.

The unearthed letter was written by Edward, the Prince of Wales — who later became King Edward VIII before abdicating the throne to marry socialite Wallis Simpson — to his own married mistress, Freda Dudley Ward, in 1919, People reported.

In the letter, Edward recalled how he lured Lord Loughborough away with the promise of a round of golf so that his younger brother Albert — the future King George VI — could be alone with Loughborough's Australian wife, Sheila, while the brothers were visiting the couple's home.

"After tea, I managed to lure Loughie [Lord Loughborough] away on the pretext of wanting to play a few more holes of golf on the local course, so as to give Sheilie a chance of being alone with Bertie [Prince Albert]," Edward wrote. "They said they were tired and we left them, but imagine my horror darling when on arriving at the links we found they were closed on Sundays!!"

He continued, "However, I kept my head and took Loughie for a walk instead!"

Simon Luterbacher, a books and manuscripts specialist at Forum Auctions in London, said, "A bit of sibling solidarity was at play as they tried to get Albert alone with Lady Loughborough," U.K.'s The Times reported.

Luterbacher described the brothers' actions as "conniving" and said it was "unusual" to discover a letter like that of Edward's that goes into personal detail.

Albert and Sheila met at a London dance in 1918. Sheila was best friends with Edward's lover Dudley Ward, and the foursome reportedly called themselves "The Four Do's."

The affair ended when the brothers' father, King George V, learned of it in 1920. The then-monarch offered to make Albert the Duke of York on his birthday under the condition that he would hear nothing further about him and Sheila. The two remained friends after their romantic relationship ended and exchanged letters.

Albert later met Elizabeth Bowes-Lyon, Queen Elizabeth's mother, and married her in 1923.

Meanwhile, Sheila eventually divorced Lord Loughborough. She went on to marry two more times, to Sir John Milbanke and Prince Dmitri Alexandrovich of Russia.

According to Luterbacher, Albert and Edward had a "strong bond at the time," but this changed after the latter abdicated the throne to marry Simpson.

Edward ascended the throne as King Edward VIII in 1936 with the death of King George V. However, he abdicated the same year in order to marry the divorcee.

"I have found it impossible to carry the heavy burden of responsibility and to discharge my duties as king as I would wish to do without the help and support of the woman I love," he said in a radio broadcast.

King George VI took over the throne, paving the way for Queen Elizabeth II to become the longest-reigning monarch in British history. Her Majesty is currently in Sandringham to mark the death anniversary of her father and 70 years on the throne on Feb. 6.

Royal biographer Alexander Larman wrote in his book, "The Crown in Crisis: Countdown to the Abdication," that Queen Elizabeth II's mother blamed King George VI's early death on the strain of being king after his brother's abdication.

"It is all but certain that the stress and pressure that he was placed under during wartime and after the abdication contributed to his premature death," Larman said.

Queen Elizabeth II will be the only British monarch to have ruled for 70 years
Queen Elizabeth II will be the only British monarch to have ruled for 70 years POOL via AFP / Oli SCARFF