A British ISIS-supporter plotted to assassinate UK Prime Minister Theresa May with a suicide bomb attack in the heart of government, a court heard on Tuesday.

The plot by Naa'imur Zakariyah Rahman, 20, to target the premier’s residence at 10 Downing Street were unravelled within days of his planned attack by an intelligence official posing as a senior ISIS leader in Syria, a jury in London was told.

Mr Rahman, of north London, had laid out his plans during discussions with an officer from the domestic security service, MI5, via the encrypted messaging service Telegram.

“I want to do a suicide bomb on Parliament. I want to attempt to kill Theresa May," he allegedly wrote in one of his messages. The following day, he said: “My objective is to take out my target. Nothing less than the death of the leaders of Parliament.”

Details of the alleged plot emerged on the first day of the trial of Mr Rahman, who was arrested in November last year.

Mr Rahman's uncle had joined ISIS in Syria and had allegedly encouraged his nephew to attack the UK before he himself was killed in a drone strike, the court was told.

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During the exchanges with the MI5 officer, Mr Rahman praised the Manchester Arena bomber, Salman Abedi, who killed 22 people when he detonated a rucksack containing explosive and metal shards in June last year, the court heard.

He allegedly said that he thought about wearing “a vest” himself before driving past parliament and pushing the button to “clear the entire block”. He added: "Everyone inside, including the Prime Minister would be dead.”

Mr Rahman has been accused of preparing terrorist acts by carrying out a recce of the site, recording a pledge of allegiance to ISIS and delivering a rucksack and jacket to be fitted with explosives.

Prosecutor Mark Heywood said he had been preparing a “full frontal assault” on the police-guarded gates that block Downing Street from passers-by – and then the door of No10 itself.

“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,” Mr Heywood said.

“In this, he expected to die. But he also hoped for personal reward beyond death and, in doing so, to cause death and great fear in a place and to people symbolic of the country itself.”

The case continues.