The son of the Yorkshire Ripper's first victim is calling for an independent investigation into whether Peter Sutcliffe was a copycat killer.

Richard McCann, who was four when his mother Wilma McCann was killed 31 years ago yesterday, spoke out on the anniversary of her death.

Wilma, the first confirmed victim of lorry driver Sutcliffe, of Heaton, Bradford, was found dead on Prince Philip playing fields in Chapeltown, Leeds.

Mr McCann, 36, said he wanted an independent inquiry to be held after hearing reports that there may have been two killers on the loose at the time of the Ripper murders.

He said he wanted the police investigation and Sutcliffe's confessions to be re-examined.

"There is nothing I want to know more than that my mum's killer is behind bars, but this leaves me with serious doubts," he said.

Mr McCann told a press conference at the Merrion Hotel in Leeds that he would be approaching his MP Fabian Hamilton (Leeds North East) to ask for his support in starting an inquiry.

He said information was missing from a 1982 report by former Inspector of Constabulary Sir Lawrence Byford which was published in June.

"I'm going to be asking why the Byford report has not been released in its entirety," he said.

"They have just released it because 25 years have passed, and they haven't released it all. I want to know why that is."

Father-of-one Mr McCann, who is a full-time student and works for the youth offending service in Leeds, said the anniversary of his mother's death was a good time to boost the awareness campaign.

He said the children of other victims were also backing the call for an inquiry.

The campaign is being co-ordinated by Noel O'Gara, an Irish landowner who believes he once employed the "real Yorkshire Ripper".

Mr O'Gara, who has written a book about the possibility that the original Yorkshire Ripper was a bearded Irishman, said the copycat theory had been pushed to one side after Sutcliffe's arrest.

"I won't let him get away," said Mr O'Gara, who added that he had a duty to draw attention to the original murderer.

A spokesman for West Yorkshire Police said: "These crimes were investigated at length by the force resulting in Peter Sutcliffe being convicted.

"The investigation was also the subject of a lengthy and detailed inquiry by Sir Lawrence Byford. There is no new evidence to substantiate these claims."

e-mail: rebecca.wright @bradford.newsquest.co.uk