Under the deal, Gary Ridgway would escape the death sentence in return for admitting to many of the so-called Green River murders that began two decades ago, the Seattle Times said, quoting unnamed sources.
Ridgway, 54, was arrested in November 2001 and was later charged with seven of the 49 Green River murders around the northwestern US city of Seattle.
But the newspaper said he was now preparing to admit to scores of Green River murders as well as to some committed as recently as 1990 and 1998 that had not been linked to the original killings.
"The additional killings represent a stunning development in a case that had been officially limited to a series of 49 deaths between July 1982 and February 1984," the paper said.
As part of the negotiations, Ridgway has told investigators that some of the deaths attributed to the Green River killer were not committed by him, the paper said, without providing a specific number.
Under the plea deal, Ridgway would admit to the 49 killings and be sentenced to life in prison without the possibility of release, the source told the paper.
Prosecutors and defence lawyers declined to comment on the reports. King County Prosecutor Norm Maleng had earlier said he would not seek a plea-bargain in the case because of the sheer number of homicides involved.
After a 19-year investigation into the Green River murders, Ridgway was arrested two years ago after the discovery of fresh DNA evidence that was not available when he was first questioned years before.
"He deserves the death penalty, but what would be the point?"
a victim's sister
Canadian officials then began looking at evidence against Ridgway in their investigation of 45 women, mostly prostitutes, runaways and drug addicts missing from Vancouver.
Sex and strangle
In many cases, the killer had sex with his victim and then strangled her.
The Seattle Times said that investigators continuing their search for victims they could link to Ridgway had earlier this year uncovered four sets of bones identified as suspected victims of the Green River killer.
Ridgway pleaded not guilty to first-degree murder in seven of the Green River slayings, and faced the death penalty if convicted.
"He deserves the death penalty, but what would be the point?" said Tim Meehan, whose pregnant sister Mary was found dead in 1983.
"Twenty years from now, when he'd actually be put to death, he'd be in his mid-70s. At least now the families have an opportunity to have answers. Closure is well worth the trade off."
The case was named after the river in or near which most of the bodies were found. Most of the victims were prostitutes working around Seattle's SeaTac Airport.