A 49-YEAR-OLD man was arrested last night by police investigating the so-called Wearside Jack hoax letters and tape sent to the Yorkshire Ripper inquiry team.

West Yorkshire Police said the man was arrested in the Sunderland area on suspicion of attempting to pervert the course of justice.

He is being interviewed at a police station in West Yorkshire.

The development comes more than 25 years after the hoax that allowed the Yorkshire Ripper to carry on killing.

The hunt for the hoaxer has been one of the North-East's most intriguing crime mysteries and has spurned a host of theories, including the claim that he was a policeman.

In 1978 and 1979, Wearside Jack, as he became known, sent police three letters and an audio tape claiming to be the serial killer.

Police were convinced he was the Ripper and switched their inquiries to the North-East for 18 months, during which time Peter Sutcliffe killed three more victims.

The murder hunt focused on Sunderland - particularly the Castletown area, where hundreds of men came under suspicion.

Meanwhile, Sutcliffe claimed three more victims, including 20-year-old Middlesbrough student Jacqueline Hill, before he was caught in Sheffield in 1981.

Last night's dramatic move comes months after Ms Hill's mother, Doreen, called for a full inquiry into claims that a hoax by a former police officer allowed the murderer to carry on killing.

For almost 25 years, Ms Hill's mother, from Ormesby, Middlesbrough, has stood firm in her belief that her daughter would still be alive if it was not for the hoaxer.

Sutcliffe had been interviewed by police, but he was discounted because he did not have a Wearside accent.

Dick Holland, who was second-in-command of the Ripper inquiry led by West Yorkshire Police's assistant chief constable, George Oldfield, said the former police officer, who was born and grew up in the North-East was "the best suspect we ever had".

He said Austin Haywood, the deputy chief constable of West Yorkshire Police at the time, was "absolutely certain" the man was the author of the "I'm Jack" hoax letters and tape.

During the Ripper inquiry, thousands of men in the North-East were interviewed, their handwriting was analysed and their voices compared with the tape.

They were also asked for their blood type, as the letter writer was of the rare B secretor blood grouping.

Mr Holland said the former police officer was eliminated because he was not the serial killer and because three characters of the alphabet as written in his police notebooks or other documents did not match those in the hoax letters. Another theory was that Yorkshire Ripper and Wearside Jack may have joined forces to carry out the unsolved murder of a prostitute during the 1970s.

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

Sutcliffe, who killed 13 women, has always denied murdering the mother-of-two 27 years ago. Her killer has never been found.

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

Mr Lavelle 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.