A 16-year-old boy talked on extreme right-wing chat forums about making homemade guns and praised perpetrators of mass shootings, a court heard.
The teenager, who cannot legally be named, discussed making a live gun from a blank-firing one, jurors were told.
Prosecutor Matthew Brook said the defendant believed in the "twisted ideology of Nazis and white supremacy".
The boy, from Rugby, denies engaging in the preparation of terrorist acts between 20 July and 3 September.
He also pleaded not guilty at Birmingham Crown Court to nine counts of possessing a document containing information useful for a terrorist about homemade firearms.
Opening the case against him, Mr Brook said the teenager had told a right-wing chat forum "the fuhrer" had "given me the go-ahead to start the first UK cell".
He had told like-minded people he was prepared to act as an "independent unit" and was "ready to take action", he said.
"In this case, the evidence will prove that he became radicalised so he fully believed in extreme right-wing ideology."
The defendant believed an ideology which thinks "its followers should bring about a race war, should accelerate its start, so that the white race can become supreme", the prosecutor said.
He added: "He came to believe in an ideology which praises terrorists who carry out mass shootings, like the Christchurch shootings in New Zealand, and called the perpetrators of such massacres 'saints'."
The jury was shown censored footage of last year's mosques attack in Christchurch, in which 51 people were killed.
They were told the defendant had a recording of the killings on his phone in "graphic detail".
The trial continues and is expected to last four weeks.
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