THE Yorkshire Ripper's reign of terror almost certainly extended beyond the 13 murders for which he was convicted, a secret report has concluded.

An inquiry said there had been an unexplained lull in Peter Sutcliffe's criminal activities between 1969, when he first came to the police's attention, and the first officially-recognised Ripper assault in 1975.

The investigation was carried out by former inspector of constabulary Sir Lawrence Byford but the report was kept secret for 25 years.

The 160-page document dating from 1982 was released by the Home Office last night after the conviction of John Humble for the notorious Wearside Jack hoax earlier this year.

At the time, the report was condemned as a cover-up by MPs.

Sir Lawrence's report said: "We feel it is highly improbable that the crimes in respect of which Sutcliffe has been charged and convicted are the only ones attributable to him.

"This feeling is reinforced by examining the details of a number of assaults on women since 1969 which, in some ways, clearly fall into the established pattern of Sutcliffe's overall modus operandi.

"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."

Sutcliffe has been questioned over a number of unsolved murders and linked to many others, including the murder of a Darlington art gallery assistant in 1977.

The report would have made uncomfortable reading for the police at the time.

It details the lapses that left Sutcliffe free to continue his killing spree, including:

* A letter sent by one of Sutcliffe's friends urging them to take a serious interest in the Bradford lorry driver, which sat in a letter tray for two months until the Ripper was arrested on January 2, 1981;

* Police failures meant officers failed to connect vital clues that could have led to Sutcliffe being arrested nearly four years earlier than he was;

* Officers who stopped him in 1977 failed to check the tyres on his car, which would have linked him to the murder of Irene Richardson nine months earlier;

* The incident room had a backlog of 36,000 documents;

* Senior police officers lacked the flexibility of mind to speedily address failures in their systems;

* By the time detectives interviewed Sutcliffe for the tenth time, the report says "even experienced and competent detectives could no longer see the wood for the trees";

The report also criticises the Wearside Jack hoax as "a red herring of mammoth proportions".

Sutcliffe was eliminated from the inquiry at one stage because his handwriting and voice did not match those of the hoaxer

Last year, hoaxer John Humble, from Sunderland, now 50, was tracked down following DNA analysis of saliva taken from an envelope seal.

In March, he admitted four counts of perverting the course of justice and was jailed for eight years.

Lorry driver Sutcliffe was sentenced to a minimum 30-year jail term for the murders of 13 women and the attempted murder of seven others between 1975 and November 1980. His final victim was student Jacqueline Hill, who lived in Ormesby, Middlesbrough.

The Byford report made a number of recommendations, including standardising procedures in major incident rooms and ensuring that one senior officer was in overall command of large investigations, even across force boundaries.

Sir Lawrence also recommended introducing computer technology to assist major inquiries after identifying a series of shortcomings in the paper indexes used by the Ripper squad.