He added, "I'm not accusing anybody of murder, but I do challenge Courtney to finally take a polygraph. I will pay for it to clear her name. She's been accused of being involved in this for so many years. If she passes, I will buy a billboard in Times Square and say on the billboard, 'Courtney Love is innocent.'"

"And I still encourage her as a grieving widow to get on the trail and to find the killers. I am not going to stop until I find the killers. I'm saying, look, her role is certainly suspicious based on the plethora of evidence that I outline in the new book. But I'm not accusing her of murder. A lot of people have doubted her. A lot of media, a lot of people close to her, openly accuse her of murder - I am not."

Ian's book includes a claim Kurt, who had daughter Frances Bean, 30, with Hole singer Courtney, did not write all the lines in his suicide note. He added "The five final lines of the suicide notes were not in Kurt Cobain's handwriting."

"That's the only section of the suicide note that deals with suicide. And it was in a completely different handwriting. I had it examined, years ago, by two of the top handwriting experts in the world. They both concluded unwaveringly that it was not in Kurt Cobain's handwriting."

Troubled heroin user Kurt was found dead aged 27 in a greenhouse room above the detached garage of his home in the Lake Washington neighbourhood of Seattle on April 8, 1994, and was holding the shotgun that killed him. The coroner and Seattle Police Department declared the death a suicide after Kurt spent years struggling with life in the spotlight.

Nirvana's ex-manager Danny Goldberg, 72, has dismissed claims Kurt was murdered, telling The Independent newspaper they were "ridiculous" as he had watched the rocker's descent into depression.