The boys, who are now 15 and cannot be named because of their age, were arrested before they could put their plans into action, Greaney said. The pair deny conspiracy to murder.

Greaney opened the prosecution case by explaining to jurors that 12 students and a teacher were shot and killed in the Columbine high school attack, carried out 19 years ago by Eric Harris and Dylan Klebold.

He said Harris and Klebold “became anti-heroes” and there was a “strange, dark subculture of adoration for them and their crimes” on the internet.

The two defendants became fascinated with the killers, Greaney told jurors. “Their interest in them turned to hero-worship,” he said.

The older boy is also accused of using a knife to carve his name into his girlfriend’s back. He denies a count of unlawful wounding.

He was the leader of the two plotters and made “significant and disturbing entries” in a diary that showed his “murderous intentions were real”, Greaney said.

The prosecutor told jurors that flammable liquids and a bag of screws were found in the boy’s hideout behind a supermarket at Catterick garrison. Here, it was alleged, the boys planned to make a nail bomb to be used in the attack.

Jurors were told the boys “sought to emulate” the Columbine killers, who described themselves as the “trench coat mafia”, by wearing long coats.

The prosecutor said researching the Columbine killers was a regular part of the older boy’s life. In the space of 14 hours last October, he allegedly made more than 30 internet searches about the Columbine attack and for instructions on how to make pipe and nail bombs, how to build a sawn-off shotgun and where to buy ammunition.

“It is plain that he was gaining inspiration from the actions of Eric Harris and Dylan Klebold,” Greaney told the jury.

The older boy was expelled from school twice between April and May last year for posting offensive images and comments about teachers on fake Instagram accounts, the jury heard.

The following month, their plans grew more serious, jurors were told, when the pair downloaded a bomb-making guide called the Anarchist Cookbook, which included detailed instructions for making a nail bomb.

In text messages later in May, the younger boy told his co-accused: “I can’t be bothered any more.” The older boy replied: “Why not take some others out as well? If you’re gonna kill yourself, shoot up the school.”

Greaney said: “What was happening in May 2017 was the development of ideas and of a relationship that was to form into a plot to attack the school that both defendants attended and to kill students and teachers.”

The older boy met his then 14-year-old girlfriend in June 2017, the jury heard, and she soon became dependent on him and he began to control her. Then, the girl told police, “the darker stuff started” and she became afraid of him.

She discovered that he was posting videos of live suicides on social media and that he felt empowered when people were frightened of him, jurors were told.

The pair discussed “scarification” and she let him carve his name into her lower back with a penknife in his hideout, an act she later described to police as “the biggest mistake I’ve ever made”.

The girl said the defendant repeatedly sought access to the shotguns her father kept for hunting and, in text messages in August last year, said: “OMG … I’m really happy” when she falsely told him she had access to her father’s shotgun cupboard. “It was no teenage fantasy. It was real,” Greaney said.

The girl’s parents stopped her from contacting him when they became deeply concerned about his malign influence, jurors were told. In a diary entry, the older boy wrote: “Her parents need to die. I hate them so f****ing much it’s insane.”

A further entry described how he planned to kill them, jurors heard: “I’ll sneak in at night, tie them up and torture them. I’ll cut em up and rip out their f***ing guts.”

Days later, the boy entered the girl’s home shortly after midnight, carrying a large kitchen knife with “LOVE” written on it and dressed as Harris, the Columbine killer. The prosecutor alleges that he intended to cause her parents “serious harm” but was startled by her mother and ran. “He had, almost certainly, expected to be able to attack as they slept,” the prosecutor said.

Text messages between the boys showed how the plot to attack their school developed. In one, the younger boy told the other: “I really wanna shoot up the school” and “I think it’ll be so fun”.

The younger boy also told a classmate about the murder plan and told her in a text message: “No one innocent will die, we promise. We’re not doing it for a while anyway because we don’t have guns atm [at the moment].”

The classmate alerted her teacher to the texts, the court heard, prompting the younger boy to confess to his vice-principal.

The trial continues.