A radical Islamic preacher who spent four years in a UK jail for soliciting murder and hate speech has made a video plea for Islamic State (Isis) to spare the life of British hostage Alan Henning.

Abdullah el-Faisal, who is said to have played a part in radicalising the shoe bomber Richard Reid and the 7/7 bomber Germaine Lindsay, is one the most radical English-speaking preachers to publicly call for Henning’s release.

The one-time imam of south London’s Brixton mosque has been a fervent online champion of Isis, even supporting the kidnapping and rape of Yazidi minority women in Iraq.

In the video released on Wednesday morning, Faisal begged for Henning’s life and said that to kill him would be against Islamic law.

There are growing signs that the kidnap of Henning is controversial even among Islamic extremists who have been sympathetic to Isis’s tactics. Faisal’s intervention is the clearest example so far. With previous kidnappings some Isis-supporting scholars have made the case that they could be spies but, according to reports, Isis’s own sharia court has cleared the aid worker of any such charge.

In the 2 min 40 sec video the Jamaican preacher, who was jailed in 2003, says: “This is a message to my brothers in the Islamic State. It is well known to the world that you have a hostage by the name of Alan Henning and that his life is in jeopardy.

“I’ve spoken to many people about Alan Henning and everyone spoke well about him. He loves people. He’s a good Samaritan. He’s a philanthropist. He is a sympathiser to Muslims.”

Sitting behind a desk with the Qur’an in his hands, Faisal continues: “So to kill someone like that, an aid worker, is un-Islamic … the sharia law doesn’t allow you to kill someone like Alan Henning.

“Because Alan Henning is not antagonistic towards Muslims, it is un-Islamic to kill such a person … Therefore I plead, I beg of you in the name of God to set this good Samaritan free.”

The video came after a Henning’s wife pleaded on Tuesday for her husband to be released.

Henning, 47 was kidnapped by Isis militants in Syria in 2012 when his aid convey was stopped by masked gunmen and he was forcibly separated from his party.

“I have seen Muslims across the globe question Islamic State over Alan’s fate,” Barbara Henning said in a statement.

Alan Henning, who is being held hostage by Islamic State militants. Photograph: Family Handout/PA

“We are at a loss why those leading Islamic State cannot open their hearts and minds to the facts surrounding Alan’s imprisonment and why they continue to threaten his life.”

Barbara Henning said she had been told that her husband had been found innocent of being a spy by a sharia court. “I implore Islamic State to abide by the decisions of their own justice system. Please release Alan.”

Earlier this month, the Guardian recorded a live online talk in which Faisal condemned others for not supporting Isis. During his 45-minute talk delivered from Jamaica, Faisal was heard saying: “Instead of embracing the Islamic State, supporting the Islamic State and doing everything humanly possible for the success of the state … all they do is slander the mujahideen,” he said.

The scholar, who has a reputation for describing many Muslim groups as heretics who should be condemned to death, recently tweeted that Isis was a punishment from Allah for “all fake Muslims who don’t want Islam”.

He has described women captured by Isis militants during the conflict as the “spoils of war”.

Towards the end of the video, which appears to have been filmed on a mobile phone, Faisal warns Isis of the consequences of killing Henning: “Killing him has no blessing, it will backfire on the Islamic State in a very bad way, the damage will not be able to be repaired.”