Matthew Brook of No5 Barristers’ Chambers has opened the prosecution of a teenager accused of planning an Islamic State-inspired terror attack on the day of a Justin Bieber concert in Cardiff.

The 17-year-old boy, who cannot be named because of his age, was detained at his home by police on June 30 after posting a picture of Cardiff Castle on Instagram.

Officers found a claw hammer, a kitchen knife and a ‘martyrdom letter’ in his school rucksack.

Opening the case against the teenager, who is from a white British background, No5 barrister Matthew Brook said a copy of a poster relating to the concert was found on the defendant’s computer.

Mr Brook told the jury at Birmingham Crown Court that the boy had conducted a number of web searches on June 28: “At 10.10pm there is this defendant searching ‘Justin Bieber Cardiff 2017’.

“Bieber is a well-known pop star and was having a large concert in Cardiff on June 30, two days after this search.”

Mr Brook said a note found in the boy’s bedroom read: “I am a soldier of the Islamic State. I have attacked Cardiff today because your government keep on bombing targets in Syria and Iraq. There will be more attacks in the future.”

Mr Brook told the jury: “In this case, the evidence will prove that he became radicalised over the internet.

“He had terrorist material stored on his computer, he published posts on Instagram which encouraged terrorism, and he was planning a ‘lone wolf’ style attack in the name of Islam.”

The defendant, from Rhondda Cynon Taf, denies two counts of encouraging terrorism by posting images on Instagram and two counts of possession of a record of terrorist information, namely an Isis propaganda magazine.

The trial continues.

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Matthew Brook is a member of the Crime Group at No5.