The only known letter to have been written on board the Titanic on the day the liner struck an iceberg could fetch as much as £100,000 when auctioned in Devizes later this month.

Written by survivor Esther Hart, but never sent, the letter had been intended to be delivered to Mrs Hart's mother in Chadwell Heath in Essex at the end of what would have been the Titanic's return journey to Southampton in April 1912.

The night the letter was written - Sunday April 14 1912 - the New York-bound Titanic struck an iceberg before later sinking with the loss of more than 1,500 lives, including that of Esther Hart's husband, Benjamin.

But Mrs Hart and her seven-year-old daughter, Eva, were among those rescued, with Eva living until 1996 and becoming one of the most-celebrated of the Titanic survivors.

The letter, and its envelope, were acquired around 20 years ago by a man who has now passed it on to Henry Aldridge & Son auctioneers of Devizes, with the letter coming under the hammer at the company’s next sale of Titanic memorabilia on Saturday April 26.

The letter is embossed with the White Star Line flag and is headed On Board RMS Titanic and dated "Sunday afternoon".

Mrs Hart describes how she had been sick the day before and unable to eat or drink but she had recovered and she and Eva had been to a church service that morning.

Mrs Hart writes of how Eva had "sang (sic) so nicely" the hymn Oh God Our Help In Ages Past. Poignantly, she talked of how she and Eva were due to sing in a concert on board "tomorrow night".

Commenting on the fact that the Titanic was not supposed to roll, Mrs Hart says: "Any how it rolls enough for me." Also she adds: "Well, the sailors say we have had a wonderful passage up to now."

At the foot of the letter Mrs Hart left space for Eva to scrawl: "Heaps of love and kisses to all from Eva".

Years later, Eva recalled how her mother had found the letter in the pocket of Mr Hart's sheepskin-lined coat after the rescue.

Andrew Aldridge, head of Titanic and ocean memorabilia at Aldridge auctioneers, said: "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."