Mark Fellows, 38, is being treated for serious injuries after he was reportedly slashed with a razor blade in maximum security jail HMP Whitemoor in Cambridgeshire.
After being attacked he was airlifted to hospital where his condition is unknown.
Fellows, also known as ‘Iceman’ for his ability to keep a cool head, was jailed for life for killing Salford gangsters Paul Massey and John Kinsella.
The attack on him is believed to be linked to the killings, The Sun reports.
Notorious Kinsella had been an influential figure in the underworld with a fearsome reputation that was once called on to help former Liverpool and England footballer Steven Gerrard.
Gerrard was being tormented by a man named ‘The Psycho’ until Kinsella intervened, it is claimed.
Fellows first gunned down Massey with an Uzi machine gun outside his Salford home in July 2015.
Dad-of-five Massey, 55, was blasted at 18 times as he raised his hands in defence and dived for cover behind bins, a jury heard during the seven-week trial.
He died within minutes.
Three years later, on May 5 last year, Massey’s friend and gang associate, John Kinsella, 53, a martial arts expert and mob enforcer from Liverpool, was murdered by Fellows in a second “cold-blooded” execution.
Massey and Kinsella’s loyal associates will not stop trying to hurt Fellows, according to reports.
A source told The Sun: “These sort of people have long memories and are willing to wait.
“Sooner or later he’ll be vulnerable, and that’s when they will pounce.”
A judge described Fellows as a “gun for hire”.
He said: “I have never had to deal with a contract killer of your kind before. There are few judges who have.
“Just punishment in your case requires you to be kept in prison for the rest of your life.”