Hundreds of sermons preached by the Islamist cleric who inspired the men who carried out the terrorist attacks in Streatham and on London Bridge in November are available on a website that allows his followers to communicate in private chatrooms, the Observer has established.

Abdullah al-Faisal, who was born into an evangelical Christian family in Jamaica and spent many years in Britain, has become one of the most successful propagandists for al-Qaida and Islamic State.

Sudesh Amman, 20, who was shot dead by police last week after coming out of prison for terrorism offences, had copies of Faisal’s speeches, the Times reported.

The newspaper also said that Usman Khan, 28, who fatally stabbed two Cambridge graduates near London Bridge last November, had the cleric’s number in his mobile phone when he was arrested over his links to a plot to bomb the London Stock Exchange in December 2010.

Others influenced by Faisal include the shoe bomber Richard Reid; at least two of the 7/7 bombers – Mohammad Sidique Khan and Germaine Lindsay; Dhiren Barot, who was jailed for trying to blow up the New York Stock Exchange; the University College London student Umar Farouk Abdulmutallab, who tried to blow up an aircraft on Christmas Day 2009; and Mohammed Chowdhury, who was part of a cell that sought to assassinate Boris Johnson and two rabbis.

Although Faisal is under arrest in Jamaica, awaiting extradition to the US on terrorism charges, his influence endures. In December two sympathisers were convicted for spreading his speeches by publishing them on a website called Authentic Tauheed. Some of the speeches glorified terrorist organisations including al-Qaida and Isis and encouraged listeners to commit or prepare for acts of terrorism, the Old Bailey heard.

They also earned Faisal money. Last year a follower in Singapore was jailed for sending Faisal $1,145 after being inspired by his sermons, which he had accessed online.

The website carries more than 700 of Faisal’s sermons, something that experts trying to combat Islamist narratives say presents society with acute challenges.

“This is an ideological battle and he is offering views which need to be countered,” said Dr Paul Stott, research fellow at the Henry Jackson Society. “We can’t censor or hide this stuff away. Once it exists on PDF or video it’s never going to be completely erased.”

The Authentic Tauheed website supports a platform called Paltalk that allows users to communicate in private.

“This needs to be taken down; this is dangerous,” said Haras Rafiq, CEO of Quilliam, the counter-extremism thinktank, who said the site also hosted speeches from Anwar al-Awlaki, an al-Qaida recruiter who was killed in a US drone strike in 2011.

Faisal was not considered a threat when he started preaching in the UK about the turn of the millennium.

He's offering a polarised world view … He gave people answers Dr Paul Stott, researcher

“Some of that goes back to the way in Britain we respond to things that we are not sure about,” Stott said. “Often we joke about it or almost dismiss it.”

Stott said, Faisal’s appeal was based on three factors.

“One, he was a convert; two, his colour – he was black and he could reach out to people who were unhappy with their status; three, he’d been to Saudi Arabia and studied there. He had that degree of legitimacy that perhaps somebody born and brought up in the UK didn’t have.”

Faisal, who was born Trevor William Forrest, was jailed in the UK in 2003 for three counts related to soliciting murder and for stirring up racial hatred. He was released in 2007 and deported to Jamaica but he has continued to inspire younger followers, including in Kenya where he has spent some time.

A dossier compiled by the Anti-Defamation League suggests that he has used the Syrian conflict to groom the next generation of western youth into supporting Isis. Before his most recent arrest he regularly took to Twitter to praise the terrorist organisation claiming “any Mujahid [fighter] who doesn’t give his bayah [allegiance] to Dawla [Islamic State]is a fake Jihadi and his death will be in vain”.

In 2014 Faisal gave a lecture describing Isis’s creation of the caliphate as the fulfilment of a prophecy.

“He’s offering a very polarised world view,” Stott said. “Removing Faisal from the country doesn’t seem to have stopped him having some influence. He gave people answers.”

In 2017 the ADL reported that Authentic Tauheed has published nearly 200 posts on topics that it considers are “antisemitic, anti-Christian, anti-American and anti-gay, to blatant endorsements of the Islamic State”, most of them from Faisal.