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An extremist who plotted a suicide attack on British Prime Minister Theresa May wanted to decapitate her in a bomb and knife assault, a court heard Tuesday.

Naa'imur Zakariyah Rahman, 20, allegedly wanted to make "big news" by storming Downing Street as May spoke outside her office.

Rahman was fully prepared to die in the attack, England's Old Bailey central criminal court in London heard.

He was arrested in November last year.

"Before his arrest prevented it, he was, he believed, just days away from his objective, which was no less than a suicide attack, by blade and explosion, on Downing Street and, if he could, upon the Prime Minister Theresa May herself," said prosecutor Mark Heywood.

In a chat with an undercover security service agent on the Telegram messaging app, Rahman said: "I want to do a suicide bomb on parliament. I want to attempt to kill Theresa May."

Rahman continued: "My objective is to take out my target. Nothing less than the death of the leaders of parliament."

He told an undercover police officer that he would make a "10-second sprint" for the door of 10 Downing Street, with his main objective to "take her head off".

Rahman carried out reconnaissance around the entrance to Downing Street, the court heard.

He gave a rucksack to the undercover police officer to be fitted with explosives, jurors were told.

The defendant is also accused of helping his friend Mohammad Aqib Imran, 22, join the Islamic State jihadist group in Syria by recording a sponsorship video.

Heywood said the two men shared the "warped ideology" of Islamic State.

Rahman denied two counts of preparing terrorist acts.

Imran has pleaded not guilty to preparing terrorist acts and possessing a terrorist document entitled "How To Survive In The West - A Mujahid's Guide 2015".