1,500 passengers and crew lost their lives in the tragedy, including Mrs Hart's husband Benjamin. The letter, headed On board 'Titanic', comes complete with an envelope embossed with the White Star Line flag. The items will go under the hammer at Henry Aldridge & Son Auctioneers in Devizes, Wilts.
In the letter Mrs Hart, who was travelling with her daughter and well-known survivor Eva Hart, talks about being ill while on board the New York-bound liner.
Mrs Hart writes: "My Dear ones all. As you see it is Sunday afternoon and we are resting in the library after luncheon. I was very bad all day yesterday could not eat or drink and sick all the while, but today I have got over it."
She goes on to describe how she had been to a church service with Eva and to talk about the trip so far.
"Tho they say this Ship does not roll on account of its size. Any how it rolls enough for me, I shall never forget it. It is very nice weather but awfully windy and cold ...
.. They say we may get into New York Tuesday night but we are really due early Wednesday morning, shall write as soon as we get there", she writes.
The letter was due to be delivered to Mrs Hart's mother in Essex when the ship returned to Southampton, but the tragedy meant it was never sent. Mrs Hart and Eva were among those rescued by HMS Carpathia.
A biography by Eva Hart, which is also due to go under the hammer alongside the letter, recalls the moment her mother later found the letter in the pocket of her husband's coat.
Andrew Aldridge, from Henry Aldridge and Son Auctioneers, said the letter was expected to fetch between £80,000 and £100,000.
The only surviving letter believed to have been written on board Titanic is expected to fetch up to £100,000 when it goes under the hammer in Wiltshire this afternoon.
The letter, written on Titanic stationery, was written by second class passenger Esther Hart just hours before the liner struck an iceberg on Sunday April 14, 1912.
Memorabilia from the Titanic will be sold at auction in Wiltshire today to mark the 102nd anniversary of the loss of the liner. The collectables will include the only known letter written on Titanic stationery. It's thought it could fetch up to a hundred thousand pounds.
The only known letter written on Titanic stationery on the day the liner hit an iceberg is to be sold at auction in Wiltshire.
Henry Aldridge and Son are holding an auction of Titanic collectables to commemorate the 102nd anniversary of the loss of the Royal Mail Steamer on April 26th.
One of the star lots is only known letter written on Titanic stationery on the day the liner hit an iceberg.
Estimated at £80000-£100000 this unique piece, complete with its envelope embossed with the White Star Burgee, was written by Second Class passenger Esther Hart, the mother of the famous survivor Eva Hart, and it is featured in Eva Hart’s biography “Shadow of the Titanic”.
She was travelling with her parents, Esther and Benjamin, a master builder, to Canada, where they were to start a new life.
Sailor-turned documentary maker Des Cox owns one of the world's biggest maritime archives: a celebration of the golden years of British shipping.
When he was handed reels of film from the 1920s - film that was about to be thrown away - he expected it to come to nothing. Not least because the film was in an extremely fragile state.
Then - as he inspected the frames - he saw an image that looked familiar. It looked like The Titanic. A lot like The Titanic. It led to the most important discovery of his career. John Ryall went to meet him at his home in Saltdean near Brighton.
People will be given the chance to experience what it would have been like when the Titanic sank, with a full size replica being built in a Chinese theme park.
The latest 6D technology will be used to recreate the sinking feeling, along with water rushing in and the ship breaking.
Around £100 million will be spent on creating the replica, which could spark rows with families of the thousands that died, including the 500 people from Southampton.
It will be built along the Qi river in central Sichuan, with construction being completed by 2016.
A violin, believed to have been played as the Titanic sunk, has been sold at auction in Devizes for £900,000.
Bidding started at just £50. It eventually sold for three times the guide price after fierce bidding from two buyers. The winner is unknown.
Wallace Hartley has become part of the ship's legend after leading his fellow musicians in playing as the doomed vessel went down, most famously the hymn Nearer My God To Thee.
Hartley and his seven fellow band members all died in the tragedy in April 1912 after the liner sailed from her home port of Southampton, 1,500 people died after the ship hit an iceberg.
His violin, which had been a gift from his fiancee Maria Robinson, was apparently found in a case strapped to his body when it was recovered from the icy Atlantic waters.
Its re-emergence in 2006, when it was reportedly discovered in an attic in Yorkshire, prompted heated debate over its authenticity.
Titanic specialist auctioneers Henry Aldridge and Son insist nearly seven subsequent years of research and tests have proved it to be the genuine article.
Now the violin - accompanied by a leather luggage case initialed W. H. H. - is being put up for sale along with a host of items from the ship at the public auction in Devizes, Wiltshire.
Andrew Aldridge, a valuer with the auctioneer, said it was likely to break the world record fee for a single piece of memorabilia from the Titanic.
The violin has a reserve price of between £200,000 and £300,000 but is expected to fetch as much as £400,000, he said.
The Titanic cemetery, where many people who died in the tragedy from Southampton are buried, is to be given a major upgrade next spring.
Headstones will be cleaned, new paths will be laid and the inscriptions will be whitened at the cemetery in Halifax, Canada.
Thousands of visitors go to the site every year.
Halifax was the nearest major city to where Titanic sank. Nearly 1,500 people lost their lives when the ship went down.
The Fairview Cemetery is one of three in Halifax, where 150 of those who died are buried
Video. A violin it is claimed was heroically played by Wallace Hartley, the Titanic band leader, as the ship sunk. It will go on show this week ahead of it being auctioned in Wiltshire next month. Forensic experts in Oxford helped a six year investigation trying to prove it is genuine.
But now relatives of another band member who also played a violin say they do not believe it is genuine. Mike Pearse reports.