THE Yorkshire Ripper and the infamous hoaxer Wearside Jack may have joined forces to carry out the unsolved murder of a prostitute during the 1970s.

A perverse loyalty between the two notorious figures could hold the key to the killing of Preston call-girl Joan Harrison.

Peter Sutcliffe, who killed 13 women, has always denied murdering the mother-of-two in Preston 27 years ago and her killer has never been found.

Neither has the infamous hoaxer, thought to come from Sunderland, who sent an audio tape and three letters to police in 1978 claiming to be the Yorkshire Ripper.

Author and former Northern Echo journalist Patrick Lavelle, who has spent five years investigating the identity of Wearside Jack, says his evidence shows Sutcliffe killed Joan Harrison but denied the murder because the hoaxer was with him at the time.

Wearside Jack then repaid Sutcliffe by sending the "I'm Jack" hoax tape to detectives to divert attention from his friend, Mr Lavelle says.

Mr Lavelle, a graduate in criminal justice studies and criminology, believes Wearside Jack then went on to kill and is responsible for the death of Sunderland prostitute Julie Perigo in 1986, itself a long-standing unsolved murder.

In his new book, Shadow of the Ripper, published by Blake, Mr Lavelle details his reasons for the link between Sutcliffe and Wearside Jack and unearths a wide range of evidence to back his claims.

Mr Lavelle said: ''Peter Sutcliffe, who admitted killing Josephine Whitaker and Irene Richardson, also killed Joan Harrison but would not admit her murder for several reasons.

''The most important reason is that he did not wish to reveal that he had a mate or accomplice who travelled with him in his cab to Preston, who witnessed the murder and had sex with Joan Harrison before she was killed. That mate was Wearside Jack."

Northumbria Police yesterday said that all aspects of the hoaxer inquiry were the responsibility of West Yorkshire Police and added that one man had been questioned in September over Julie Perigo's murder, but had been released without charge.

A spokesman for West Yorkshire Police said: ''All we can reiterate is that much effort was put into this aspect of the inquiry at the time of the original investigation, to no avail.

''Without any hard evidence, there is no reason to believe any new suggestions will be any more accurate than those we checked out at the time.