It was found among possessions belonging to Welsh surgeon Sir John Williams, a chief suspect in the Victorian murders.
Sir John, known to his family at the time of the killings as "Uncle Jack" was the surgeon to Queen Victoria who lived in London at the time of the slayings.
He fled the capital after the murders and later founded the National Library for Wales in Aberystwyth.
One of his distant relatives has now unearthed the old black-handled surgeon's knife, which he used for operations, and believes it could be the murder weapon.
Tony Williams, 49, Sir John's great-great-great-great nephew, has now published a book, which features the startling image of the knife, to expose his relative's guilt.
He found the blade among a stash of possessions left by the Welshman, including three glass slides which contains smears of a uterus.
Mr Williams said: ''Why would he leave this behind? I am convinced that this is the knife used by Sir John Williams to murder those women.
''It is widely know that the person who carried out the killings would have had significant medical knowledge.
''Sir John Williams was an accomplished surgeon and routinely performed abortions on women. He held surgeries all over London at the time of the murders."
He added: ''Dr Thomas Bond, a pathologist who examined the body of Mary Kelly, said the ripper had used the same six inch knife in all the murders.
''He said it would have been at least six inches long, very sharp, pointed at the top and about an inch in width - a surgeon's knife.
''This is the knife that fits the description that I've held in my hand back in the National Library of Wales.''
Sir John Williams, born November 1840, was a Welsh surgeon and physician who attended to Queen Victoria.
He was raised to the baronetcy by her for his work in 1894.
The medic had a surgery in London's Harley Street the time of the murders - which saw five prostitutes butchered in the streets of Whitechapel in the city's East End.
Mary Ann Nichols, Annie Chapman, Elizabeth Stride, Catherine Eddowes and Mary Jane Kelly were killed by the ripper between August and November 1888.
They had been expertly sliced open and some had been disembowelled. Two had their uterus's missing.
Many suspects have been put forward for the gruesome killings - but it wasn't until this century that Sir John Williams was named.
Tony Williams began researching his relative a decade ago - stumbling across an archive of his old possessions in the National Library of Wales.
The surgeon helped found the institution when he moved from London to Wales after the killings.
He become its first president back in 1907, before he died in October 1926 - leaving a sum of money, books and a number possessions to the library.
Mr Williams claims the possessions - including his old diary, the knife and some glass slides - proves his ''Uncle Jack'' had the medical expertise and motive for the murders.
He said: ''I looked through the possessions that he left and found the knife along with three glass slides.
''The smears on the glass slides have been tested and are confirmed as being matter from the uterus.
''I know Sir John was obsessed by the fact that his wife, Mary Hughes, could not have children. I think that was his motive."
He added: ''I think he was a Jekyll and Hyde-type character who may have been driven to commit murder because his wife could not have children. He was also known to be working on a cure for his wife's problem.''
A number of other key pieces of evidence also point towards ''Uncle Jack'', including testimony from a witness who saw a victim on the night she died.
George Hutchinson observed Mary Kelly with a man matching the description of Sir John Williams - wearing a long dark coat with a red stone.
The surgeon's colleague Herbert Spencer wrote about the doctor and the way he used to dress - claiming he always wore a dark silk tie held by a pin set with a red stone.
In 1885, three years before the murders, Sir John Williams also claimed he performed an abortion on Mary Ann Nicholls in the infirmary of the Whitechapel workhouse.
The details were unearthed by Tony Williams in his medical notebook. He also found that many pages of Sir John's diary in 1888 were missing.
He said: ''There is compelling evidence in my view that Sir John Williams was Jack the Ripper. When I began to look into this I found a terrible truth - that I was related to this serial killer.
''The knife, his possessions and all the evidence points to it. A lot of members of our family don't really speak about it - I think they are a bit upset.
''But in my mind the case has been solved - it was Sir John Williams.''
Father-of-three Tony Williams, an author who lives in Swansea with wife Catherine, 49, has now released book, "Uncle Jack - A Victorian Mystery".
The publication includes the picture of the knife and the details of his relative's guilt.
as presented on telegraph.co.uk
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