Kharum Butt was one of the terrorists who killed at least seven people on Saturday
Khuram Butt, 27, is said to have been the ringleader behind Saturday night's devestating attack which killed seven people and injured 48.
He and his fellow attackers Rachid Redouane and Youssef Zaghba were shot dead by police in Borough Market eight minutes after their rampage began.
As more disturbing details emerged about the deranged father of two's life, anti-extremism think tank Quilliam revealed one of its senior members was attacked by Butt at a religous festival in July last year.
Dr Usama Hasan was attending an Eid celebration in east London with his nine-year-old son and elderly parents when he was aproached by the extremist, said to be an associate of jailed hate preacher Anjem Choudary.
"He [Butt] began by saying, ‘You believe we come from apes,’" Dr Hasan said.
"And then continued the verbal attack, saying things such as ‘You work for Quilliam, you take government money to spy on Muslims and work against Muslims,’ ‘You support gay marriage,’ and “How dare you come to a Muslim event you are an apostate’.”
Quilliam said Butt eventually launched himself at the scholar, and was reported to police.
It took months for officers to arrest Butt over the assault, and he was given a caution and let go.
Quilliam chief executive Haras Rafiq said: “It’s one thing when someone from the general public reports an individual as a violent extremist, but when experienced experts who are involved in deradicalising jihadists – as Dr Hasan is, report them, a caution is not enough.”
Several other people have since come forward claiming they flagged up concerns about Butt's extremism with authorities.
Saturday's attack was the third to target the UK in three months
Erica Gasparri, 42, told The Times she had reported Butt to anti-terror police over fears he was attempting to radicalise schoolchildren, after challenging him in a park near a school.
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A friend told the BBC Asian Network he reported Butt to the anti-terror hotline after he began expressing increasingly radical views and justifying terror attacks, but he was never arrested.
And Mohammed Shafiq, chief executive of anti-extremism group the Ramadhan Foundation, said Butt called him a "Murtad" – traitor in Arabic – when he confronted Choudary about supporting terrorism days after the 2013 murder of Lee Rigby.
Butt also appeared on television last year in the Channel 4 documentary The Jihadis Next Door, which looked at Islamic extremism in Britain.
He was shown unfurling the black flag of Islamic State on the show.
Police said Butt had appeared on the radar of security services but was in "the lower echelons of our investigative work".
He was reported to counter-terrorism authorities in July last year after a "violent scuffle" with a member of an anti-extremism organisation.