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. 

"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.

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

Part of Christie's Classic Week Exceptional Sale in July, the intimate document penned by one of the world's most famous composers is expected to get the highest price for a Mozart letter to date - with an estimated worth of between £300,000 and £500,000 ($380,490 and $634,150).

Mozart wrote many 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. 

As his career started to take off following the success of his first major opera, the composer's love life was "getting a bit complicated" and he was eager to bring his upcoming marriage to Constanze forward. 

"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". 

"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. 

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