In this file photo President of Sotheby's Germany, Heinrich Count of Spreti, shows a handwritten letter of composer Leopold Mozart and his son Wolfgang Amadeus Mozart at Sotheby's in Munich, southern Germany, May 6, 2008. The communication between Wolfgang Amadeus Mozart and his father through letters was extensive and showed a high value in studying the composer's life on many levels. (PHOTO / AP)

LONDON - A letter showing Austrian composer Wolfgang Amadeus Mozart in a "complete panic" over his love life is set to go up for auction at Christie's in London next month.

Written by the composer in the summer of 1782 the two-page letter shows him asking a close friend for advice following a "misunderstanding" with his soon-to-be mother-in-law.

ALSO READ: T-Rex skeleton sells for more than $6m at Swiss auction

"She's about to send in the police to retrieve his fiancee with obviously terrible consequences for his reputation," said Christie's Head of Books and Manuscripts, Thomas Venning.

Mozart wrote a lot of letters during his life, but Venning said it was rare to see one of them at auction, and rare to get this "degree of insight... into his character... and feelings at a pretty crucial moment in his life"

"He's in a complete crisis and rather wonderfully pouring out his heart in a totally unfiltered way."

Mozart wrote a lot of letters during his life, but Venning said it was rare to see one of them at auction, and rare to get this "degree of insight... into his character... and feelings at a pretty crucial moment in his life."

Mozart was 26 years old when he wrote the letter, shortly after he arrived in Vienna. While his career was taking off following the success of his first major opera, his love life was "getting a bit complicated" and he was keen to bring his upcoming marriage to Constanze forward.

ALSO READ: Freddie Mercury belongings headed for auction

"He comes from a very strict Catholic background. So the fact that he's already cohabiting with Constanze, that they're already sleeping together, he's sort of throwing caution to the wind," Venning said. "That kind of liberated nature of the way he's behaving" in both his private and professional life Venning describes as "refreshing".

READ MORE: Michael Jordan's 1992 Olympic practice jersey up for auction

"It tells you something about Mozart as a kind of revolutionary figure. He's someone who isn't obeying the old rules," he said.

Mozart and Constanze married shortly after the letter was written, and were together until his death in 1791 at the age of 35. The letter is part of Christie's Classic Week Exceptional Sale in July and is expected to get the highest price for a Mozart letter to date with an estimated worth of between 300,000 pounds and 500,000 pounds ($380,490 and $634,150).