The whistleblower who exposed a neo-Nazi plot to kill an MP has been warned repeatedly by police that he is at risk of being murdered by far-right terrorists.

Robbie Mullen has received five “Osman notices” (named after a high-profile 1998 case), credible warnings of a high risk of murder that are issued by police to the possible victim, after testifying against the proscribed terror organisation National Action.

Jack Renshaw, a National Action member from Skelmersdale in Lancashire, last year admitted a terrorist plot to murder local Labour MP Rosie Cooper with a machete. Earlier this month a jury failed to reach a verdict on whether he was a member of the banned group.

Now, in his first major interview since Renshaw’s trial ended, Mullen has spoken in stark terms of how the decision to divulge details of the murder plot “totally ruined” his life.

“The police have offered me witness protection after each death threat but each time I’ve turned it down because I want the option to go back home,” Mullen, 25, from Warrington, told the Observer. “They would have made me start again, changed my name.”

Jack Renshaw admitted making preparations to kill his local MP Rosie Cooper in 2017. Photograph: GMP/PA

Mullen, a former senior member of National Action who became revolted by its ideology, divulged the plot to murder Cooper to anti-fascist charity Hope Not Hate, details of which are revealed in a book published this week. Since then the threats have come regularly, with the first Osman warning given weeks after Mullen left the organisation and the latest “credible death threat” coming in February.

When Mullen has returned to the north-west, he has been quickly reminded of the risks. “I’ve seen a few faces in the street. I assume it’s because they’ve been on their own that they haven’t attacked me. People are cowards on their own. They just seem shocked to see me,” said Mullen, a former warehouse worker.

But he knows the capabilities of his ex-colleagues in a group that celebrated the murder of Labour MP Jo Cox by a far-right terrorist. And life on the run from them is tough.

Mullen, who joined the group after feeling socially and politically isolated, has been effectively blacklisted because of his past association and cannot secure a job because he is unable to get a clean bill of legal health from the reference agency. Similarly, he was forced to say goodbye to his former life almost immediately after he texted Matthew Collins, Hope Not Hate’s head of intelligence, at 10.40pm on Saturday 1 July 2017 to first reveal the plot.

“I didn’t want to move or leave my job but had to drop everything and everyone I knew. It has totally ruined my life,” said Mullen.

Before texting Collins, Mullen had spent several hours at a National Action meeting in the Friar Penketh, a Wetherspoon’s pub in Warrington, listening to Renshaw’s plan to kill Cooper with a 19-inch gladius machete.

Speaking in a London pub last week, the softly spoken teetotaller said: “There were five of us sat at a full table, and I wasn’t going to be the person to object to it.

“Renshaw told his plot over two to three hours. It wasn’t just a quick outline. He kept on explaining, saying he’s got the machete, that it’s designed for cutting through pig. And the pig is the closest thing to human flesh. There was an aura around the room. It was as if this is what they had been waiting for; everybody sort of had a smile on their face.”

I didn’t want to move or leave my job but had to drop everything and everyone I knew. It has totally ruined my life Robbie Mullen

Mullen left the pub and contacted Collins, who arranged a getaway vehicle and a secret hideaway in London. Hope Not Hate refused to hand Mullen over to the police until he had been promised immunity.

Collins, who handles a network of informants who have infiltrated a number of far-right groups, knows what Mullen is going through. Once an activist with the far-right terror group Combat 18, Collins became an informer and fled to Australia, returning 10 years later as a wanted man. “You spend the rest of your life looking over your shoulder. Everybody wants to be a hero.”

Both Collins and Mullen are adamant that National Action would have ultimately killed someone. Collins describes the methods of National Action, which continued to operate after being banned in 2016, as the most sophisticated far-right outfit he had encountered. “They were always chasing an evolving ideology. Towards the end it became about terrorism, very Baader-Meinhof [German far-left group], Shining Path [Maoist guerrilla group in Peru], with some sections very sympathetic to the IRA,” said Collins, author of the book, Nazi Terrorist: The Story of National Action.

The book charts the evolution of dangerous new far-right groups such as the System Resistance Network and the Sonnenkrieg Division, whose ideology is woven around Satanism, nihilism, rape and paedophilia.

Following Renshaw’s trial, it was revealed that he was a convicted paedophile and had also planned to kill the police officer investigating him for sexually grooming young boys.

• This article was amended on 21 April 2019 to correct the description of the Baader-Meinhof group from far-right to far-left.