JACK The Ripper was a Polish immigrant who died aged 54 in a Hertfordshire mental asylum - 31 years after he terrorised the dark streets of east London.

That is the belief of the senior officer who investigated The Ripper murders, original handwritten notes given to Scotland Yard have shown.

Chief Inspector Donald Sutherland Swanson never caught the killer who butchered at least five women during his reign of terror in Whitechapel in 1888.

But notes written in the margin of a book discovered by his relatives reveal the chief inspector was convinced The Ripper's true identity was Aaron Kosminski.

The document was handed over to the Met yesterday by Mr Swanson's great grandson, Nevill Swanson, 68, who said: ''My great-grandfather did believe he got his man, but he never nailed him."

The book will be kept in the Crime Museum which was reopened yesterday after months of restoration.

The museum, which is closed to the public and will be used to train officers, is home to artefacts from many of London's most notorious murders.

As one of the biggest unsolved mysteries ever, The Ripper legend spawned hundreds of conspiracy theories and inspired dozens books, documentaries and movies over the past 118 years.

The Ripper either stabbed or strangled his victims, sometimes removing their organs and leaving their bodies horrifically mutilated.

Mr Swanson wrote in the margins of the memoirs of the retired assistant commissioner Robert Anderson, "The Lighter Side of My Official Life", that the killings stopped after Kosminski realised detectives had identified him as the prime suspect.

Kosminski, who worked as a barber after arriving in London in 1882, was never interviewed because he was insane.

The only eyewitness to The Ripper murders was alleged to have identified Kosminski in a identity parade at a Metropolitan Police convalescent home in Brighton.

But Swanson wrote that the witness refused to testify against Kosminski because they were both Jewish.

Mr Swanson wrote: "because the suspect was also a Jew, and also because his evidence would convict the suspect, and witness would be the means of murderer being hanged which he did not wish to be left on his mind."

He added: "and after this identification which suspect knew, no other murder of this kind took place in London."

At the back of the book Swanson added: "Continuing from page 138, after the suspect had been identified at the Sea Side Home where he had been sent by us with difficulty in order to subject him to identification, and he knew he was identified. On suspect's return to his brother's house in Whitechapel he was watched by police (City CID) by day and night. In a very short time the suspect with his hands tied behind his back, he was sent to Stepney Workhouse and then to Colney Hatch and died shortly afterwards - Kosminski was the suspect - DSS."

However, records showed Kosminski lived until 1919 after spending the last years of his life at the Leavesden Imbeciles Asylum, north of Watford, and was buried in a nearby cemetery.

Mr Swanson's views were first made public in 1981, but chief superintendent Steve Lovelock, head of the Met's Crime Academy, said the document remained a "remarkable artefact".

"What I find most interesting is that we have the officer in charge at the time putting forward the name of the person he believed was the main suspect after he had retired," he said.

Ripper historical researcher Keith Skinner said there was no evidence against Kosminski, or any of The Ripper suspects.