A former labourer who admitted being the Yorkshire Ripper hoaxer known as Wearside Jack has pleaded guilty to four counts of perverting justice.
Humble will be sentenced on Tuesday
John Humble, 50, of Flodden Road, Sunderland, taunted detectives when he claimed to be the Ripper in three letters and an audio tape in the 1970s.
Ripper squad detectives wasted huge amounts of time and effort when they concentrated on the Sunderland area.
Humble is to be sentenced at Leeds Crown Court on Tuesday.
A confidential police document obtained by the BBC shows how officers trying to catch the Yorkshire Ripper in 1979 were told to eliminate anyone who didn't have a North-Eastern accent.
The real Ripper, Peter Sutcliffe, came from Bradford, not the North-East.
In 1981, he was given 20 life sentences for killing 13 women and attempting to kill seven more.
However, the North-Eastern accent on the hoax tape sent by John Humble in the summer of 1979 clearly sidetracked the inquiry.
The internal document - from West Yorkshire Police murder incident room, and dated Sept 1979 - listed five points for elimination from the inquiry. One of them stated 'if his accent is dissimilar to a North-Eastern (Geordie) accent'.
Paul Worsley QC told Leeds Crown Court tha Humble had had a fascination with the original Jack the Ripper, who terrorised the streets of London in 1888.
"Considerable trouble had been taken to reflect the original phrases in these letters and tapes," said Mr Worsley.
He said Humble had also been persistent with his letter writing, and did not stop, even when he knew the police were taking them seriously.
"He was to send another letter and then a tape. That made it clear he wanted to send the police off the trail of the true killer."
The real Yorkshire Ripper Peter Sutcliffe was jailed in 1981
In police interviews, the court heard that Humble had claimed that he wrote the letters to obtain notoriety for himself.
But he also said that when he called incident rooms in Bradford and Sunderland in September 1979 to tell officers his letters and tape were a hoax, no-one believed him.
When asked why he had phoned in twice he said: "Because I felt guilty."
Referring to the murder of Barbara Leach he said: "Because that lass, one of the lasses, was murdered.
"I blamed myself for it. That's why I phoned in.
"They took no notice and another two got killed."
Speaking after Humble's guilty pleas, Barbara Leach's mother, Beryl, said he deserved to be punished.
"I think it's started off as a hoax but he should have realised he was misdirecting the police and he was causing criminal damage to people.
"He is a criminal."