John Sweeney, 54, already serving life for the attempted killing of a former lover, Delia Balmer, was found guilty of murdering Americans Melissa Halstead, 33, and Paula Fields, 31, both of whom disappeared during their respective relationships with him, Britain's Sky News reported.
Authorities think he also killed three other girlfriends and two German men, The Daily Telegraph reported. If true, it would make Sweeney one of Britain's worst serial killers.
Balmer survived Sweeney's attack on her with an axe and knife.
"I'm so angry at the law, never mind him, because I told them what he would do and they let him do it," Balmer said.
"He was like a madman, he was possessed. He had a gun and my kitchen knife, kneeling on the bed while I was tied to the bed."
"Sweeney is a man with a deep-seated hatred of women, illustrated by the way he cut up and disposed of his victims," criminology Professor David Wilson said.
"The level of planning, and his age now in his mid-50s, suggests he has killed others and the police are right to look for more victims."
Police haven't given up trying to see Sweeney convicted of the additional slayings.
"As John Sweeney contemplates a life behind bars I would like to assure him that this investigation will continue as we seek to identify and trace other possible victims within the U.K., Holland and across Europe," Detective Chief Inspector Howard Groves said.