A DOUBLE rapist who turned "vigilante" behind bars and attacked notorious Scottish serial killer Peter Tobin is causing panic within the prison system over fears for the safety of inmates.

Sean Moynihan was last week told he faces spending the rest of his life in jail after raping a teenager at knifepoint on a secluded cycle path.

Since his incarceration last year, the 31-year-old has already slashed serial killer Peter Tobin, leaving him permanently scarred, and attacked rapist Ernest Radic with a metal bar.

Now prison sources fear Moynihan will become "out of control" as he faces dying in jail.

One prison insider told the Sunday Herald: "Moynihan seems to think he's a one-man vigilante mob, carrying out revenge attacks on Tobin and Radic.

"And now he's been told he might never see the light of day again, he may think he has nothing to lose and really let rip. There are real concerns he'll be out of control.

"It seems to have escaped him that he's in with these beasts because he's a beast himself. Slashing and battering folk isn't going to change that. The problem is he's intelligent and that makes him even more dangerous."

Former fitness instructor Moynihan, from Renfrewshire, was last week jailed for a minimum of nine years and given a lifelong restriction order after he was convicted of raping an 18-year-old woman on a cycle path in Johnstone, in October 2014.

It was later revealed he carried out the brutal attack after being released from a 2009 jail term for raping a 16-year-old girl.

As he was sentenced on Thursday at the High Court in Glasgow, Judge Bill Dunlop QC told Moynihan that a risk assessment of the danger he presented to public safety was "alarming and disturbing".

The judge told Moynihan: "You are what you are because you want to be. You can change, but if you do not, you face spending the rest of your days behind bars. You may well never be released, Mr Moynihan."

According to prison sources, he decided to turn vigilante the day after he was convicted on July 1 this year.

"It was the day after he was found guilty in Saughton", the insider explained. "Tobin woke up at about 12.30pm and saw Moynihan standing over him with a razor. He chibbed his face and slashed his neck. Tobin is scarred for life."

Moynihan pled guilty at Edinburgh Sheriff Court to assaulting 69-year old Tobin by repeatedly striking him on the head and body with a razor.

As Moynihan left the cell with blood on him, Tobin rang the alarm and alerted prison officers.

Moynihan told the officer: "I slashed Tobin across the throat and face."

When later asked why he had carried out the attack, Moynihan replied: "I am never getting out. Tobin has been annoying me for months with his attitude towards his offences and his refusal to tell anyone where the rest of the victims' bodies are."

Doctors said Tobin had a 20 centimetre slash running down the right side of his face and onto his throat.

In October, Moynihan admitted hitting Radic from behind with a metal bar in the Barlinnnie prison gym.

The attack on Radic - a Hungarian from Slovakia who repeatedly raped a woman and filmed it on his phone - took place in December 2014.

He was sentenced to six months in prison for the gym assault and 32 months for the attack on Tobin, both to be served alongside his current time inside.

A Scottish Prison Service spokesman said staff were "well equipped" to deal with long-sentence prisoners.

When asked specifically about Moynihan, the spokesman said: "Our job is to manage him and others around him."

Judge Dunlop stated in court that Moynihan's "intelligence" made him "dangerous", referring to his bid to "cover-up" the rape.

Prosecutors said they believed he had trawled the victim's Facebook page to back up his lies that any sex had been consensual.

Detective Chief Inspector Fil Capaldi, from Police Scotland's Public Protection Unit, said: "This was a complex and protracted investigation for the team.

"Moynihan’s sinister web of lies and deceit were unravelled as he tried in vain to cover up what he had done.

"As always, the officers dealing with this investigation were committed from the outset to bring to justice the person responsible and I would like to commend them for their tenacity and dedication.

"Any decent person will be rightly horrified at the details of this case and thankfully such crimes are extremely rare. Moynihan's sentence reflects the gravity of his crime.

"In particular, I want to pay tribute to the victim in this case for her bravery and resolve which was instrumental in bringing Moynihan to justice to answer for his crime."