LONDON — A letter from Princess Margaret to Frank Sinatra -- asking the legendary crooner to perform a love song at her home -- is up for sale, a British auction house said Tuesday.

In the two-page handwritten note, the glamorous younger sister of Queen Elizabeth invites the American singer to her Kensington Palace residence and requests he "brush up'' on the song, Out of this World.

An affectionate "Dear Frank,'' opens the letter, which is written on stationery embossed with the trademark "M'' insignia that is characteristic of Princess Margaret's correspondence.

"We would love to dine with you and perhaps it would amuse you to see our ancient dwelling which we have brought up to date,'' the March 19, 1971 letter says.

Argyll Etkin, an auction house which specializes in the sale of royal memorabilia, declined to reveal the details on how the letter came into its possession, except to say it was sold by a private organization for an undisclosed amount.

"I think it's very unusual to find a letter on the open market between two very well-known personalities. That's what makes it quite special,'' said Ian Shapiro, the auction house's joint managing director.

The letter is up for sale now for the flat price of $2,653 US on a first-come, first-serve basis.

Christopher Warwick, the princess' authorized biographer, said the letter's casual, easygoing tone is telling of Princess Margaret's close but informal relationships with her circle of friends, many of whom were in show business.

"This is a lady who in many ways was misunderstood. She wasn't this Grand Dame,'' Warwick said.

Princess Margaret was known for her cadre of prominent friends, her love of the arts and her patronage of the ballet. She was well-acquainted with celebrities such as Elizabeth Taylor, Judi Dench and Warren Beatty.

"When she had showbiz dinner parties, there would always be songs around the piano after dinner. This is something she loved,'' Warwick said.

By the time the letter was written, Princess Margaret already had an established friendship with Sinatra that possibly dated back to the 1950s, according to Warwick. The lyrics of Margaret's special request include the line: "So let me fly out of this world; And spend the next eternity or two with you.''

Princess Margaret died in 2002 at the age of 71 after suffering a stroke.

The emergence of the letter to Sinatra comes amid criticism over the sale of a 1957 portrait of the princess, which hung in her Kensington Palace home. Other items to be sold at a two-day auction in June by Christie's of London include the Poltimore tiara the princess wore at her wedding in 1960.

Her estate, which is owned by her two children, has an estimated value of $13.6 million. The hefty inheritance tax levied on the estate has prompted her son David Linley, a furniture designer, and his sister, an artist, to sell over 800 items ranging from jewelry, furniture and works of art from their mother's private collection.