Gangland hitman John Kennedy survived a murder attempt while waiting to go on trial for killing Kenny Reilly.
A cellmate tried to strangle the 41-year-old assassin in a drug -fuelled rage.
The prisoner also attempted to self-harm in the frenzy which happened in Glasgow’s Barlinnie prison.
The terrifying incident was one of three attacks on Kennedy behind bars.
A source said: “The guy who attacked Kennedy was out of his face on drugs.
“He tried to maim himself and choke Kennedy to death before the wardens stepped in.
“The prisoner was heard shouting ‘He’s a killer, what am I expected to do?’ while staff dragged him out of the cell.”
Sources say the attacker had taken psychoactive drugs known as Spice before targeting Kennedy.
The gangster was one of four men who were given life sentences for the murder of Reilly, 29, in Glasgow’s Bilsland Road in April 2018.
He was told he must serve a minimum of 26 years before he can apply for parole.
Co-accused Darren Eadie was jailed for at least 24 years while his dad Morton Eadie and Ross Fisher each got a minimum of 22 years at the High Court in Edinburgh.
Reilly was shot in the head as he sat at traffic lights in the passenger seat of a friend’s car.
The source added: “Kennedy has been described as a psycho but the guy who tried to strangle him really is one.
“The attack came out of nowhere because they were both on good terms before it happened.”
Underworld sources have said Kennedy was a “gun-for-hire” who called himself a “one-man band” before he went on trial for the murder of Reilly and Jamie Campbell.
Campbell was shot in Glasgow’s Drumchapel area in March 2006. Kennedy’s legal team implicated dead Daniel gang enforcer Kevin “Gerbil” Carroll as the killer.
Carroll’s brother-in-law Alexander Sutherland, a senior member of the crime group, told the jury that Carroll revealed he’d shot Campbell during a car journey.
A jury returned a not proven verdict on Wednesday.
An insider said: “Kennedy might have thought he was a one-man band before the trial but that won’t be the case now.
“The Daniel clan will be wanting him to do things for them while he’s locked up.”
A spokesman for the Scottish Prison Service said: “We don’t comment on individual prisoners.”
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