The Yorkshire Ripper probably committed more attacks than the murders and attempted murders for which he was jailed, a secret report says.
Peter Sutcliffe was jailed in 1981 for the murders of 13 women
The Byford Report said there was an "unexplained lull" in the Ripper's activities between 1969 and 1975.
The inquiry has been made public after being kept secret for nearly 25 years.
Lorry driver Peter Sutcliffe was jailed for the murders of 13 women and the attempted murders of seven others in northern towns between 1975 and 1980.
Sutcliffe was sentenced to a minimum of 30 years in jail for the brutal attacks on women but the 1982 report raises the possibility he may be responsible for other "assaults".
The inquiry into the West Yorkshire Police investigation into the Ripper's crimes includes details of these attacks but this part of the report, entitled "Description of suspects, photofits and other assaults", remains censored by the Home Office.
Also partly censored is a section about Sutcliffe's "immediate associates".
The Byford Report points to a "lull" in Sutcliffe's activities between the time when he first came to the attention of the police in 1969 and the first officially recognised Ripper assault in 1975.
Its author, Sir Lawrence Byford, says: "It is my firm conclusion that between 1969 and 1980 Sutcliffe was probably responsible for many attacks on unaccompanied women, which he has not yet admitted, not only in the West Yorkshire and Manchester areas but also in other parts of the country."
The report said West Yorkshire had seen a number of other murders and serious assaults during the Ripper enquiry.
"We now know that some of these cases were connected and have since been admitted by Sutcliffe, whilst others, which he has not admitted, are believed to have been his work." it added.
Sir Lawrence added that he was "sure" senior police officers were aware of the possibility that the Ripper was responsible for more attacks.
The report, released under the Freedom of Information Act, details how detectives made "major errors of judgement" during the five years it took to apprehend Sutcliffe.
It suggests police failures meant officers failed to connect vital clues which could have led to Sutcliffe being arrested nearly four years earlier than he was.
Officers who interviewed him in November 1977 failed to examine his red Ford Corsair - the tyres of which would have linked him with the murder of Irene Richardson nine months earlier.
This was a "vital error" which gave Sutcliffe the opportunity to commit a further seven murders before his apprehension in January 1981, the report said.
In 1982, a summary of the report was published, which suggested the police investigation suffered "information overload" because of the massive public response to the hoax letters and tapes sent by John Humble, who became known as Wearside Jack.
The Byford report described the hoax as "a red herring of mammoth proportions."
Humble was jailed for eight years in March for attempting to pervert the course of justice. His conviction is believed to have influenced the Home Office's decision to finally publish the Byford Report.