The only known surviving letter written by a passenger on the Titanic the day it sank is expected to fetch tens of thousands of dollars at auction.

The Henry Aldridge and Son auction house says the letter penned by Esther Hart, who survived the sinking with her seven-year-old daughter, Eva, will go under the hammer April 26.

The letter, which is mentioned in Eva Hart's biography, is believed to have been written hours before the ship struck an iceberg on Sunday, April 14, 1912, with her mother writing that Eva had sung "so nicely" at a church service that morning.

Esther Hart found the letter in her husband's jacket after she and her daughter were rescued, according to BBC News. Her husband, Benjamin, was one of more than 1,500 people who perished in the sinking. Eva Hart died in 1996 at the age of 91.

"The importance of this legendary item cannot be overstated, being the only known surviving example of its type to have been written on that fateful day, surviving the sinking, and having belonged to such a well known survivor," Henry Aldridge and Son said in a news release.  

"The letter and envelope are in remarkable condition, with an impeccable provenance, and they represent a truly unique opportunity to own the finest example of its genre in existence." 

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