More than 23,000 people already have signed a petition urging a British prison to restore a Christian volunteer chaplain who was banned by a Muslim prison official who claimed the pastor was teaching extremism.

And more are joining the cause hourly.

The petition is being run by Christian Concern, which gave the background to the startling case.

"A highly respected Christian volunteer Chaplain at HMP Brixton has been removed from chaplaincy work, and an internationally respected Christian course banned by a Muslim senior chaplain, in an attempt to wipe out 'Christian domination' at Brixton prison," the organization reported.

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The chaplain, Paul Song, relocated from South Korea in 1992 and began working as a minister at London Shepherd Church in 1996. In 1998, he began using his one day off each week to volunteer alongside 15 other Christians at HMP Brixton, and he was taken on as a chaplain.

Among other services was his provision of a program called Alpha, which introduces spiritual seekers to Christian faith.

However, when Mohamed Yusef Ahmed, a Muslim, took over as senior chaplain in 2015, things started going wrong.

Song explained: "Imam Mohamed's discriminatory agenda was clear from the outset. He began scrutinizing the material for each of our courses, commenting that the material was 'too radical,' and that the Christian views expressed were 'extreme.' He paid scant regard to the fact that the courses are mainstream Christian courses, used by churches throughout the world. He also said he wanted to 'change the Christian domination within HMP Brixton.'"

Even though there were no prisoner complaints about Song, things came to a head last fall when Mohamed told him: "You do not have permission to enter the wings and nor do you have the permission to speak to any prisoners here at HMP Brixton. If you do turn up to here without my prior permission from me, your keys will be confiscated and you will be walked to the gate."

Weeks later, Song was accused of calling a prisoner a "terrorist," which he denies. And the prison has refused to provide any evidence for the accusation.

Song said the "whole reason I served at the prison was because of my desire to bring the good news of the gospel to people, regardless of their religion or background."

"I believe that it has the power to transform the lives of all who believe, and so I would never do anything which may cause an individual to not want to hear the Christian message."

He continued: "The prisoners who attended the Christian courses were of different religions, sexual orientations and ethnic backgrounds. Some of them were violent and held views which strongly opposed mine, yet I never made any judgmental or stereotypical comments to any of them. For 19 years, I served with an exemplary record. I worked alongside the prisoners and other staff members in harmony, recognizing our differences and praying that they would come to faith, but equally respecting their decisions and background. I would have had plenty of opportunities to make offensive comments should I have wanted to, and yet during this time, no complaints were ever made about me."

The Christian Legal Centre's Andrea Williams said: "To call this Christian who has served without a blemish for almost 20 years an extremist defies belief. Pastor Song's work with prisoners has been shut down for no good reason, denying prisoners of life-giving ministry."

The organization is considering what legal steps it might take.

Meanwhile, there is an online petition signed by more than 23,000 urging that Song be reinstated as a chaplain.

The Christian Institute notes prisoners have been speaking up in Song's defense, including former Brixton prisoner Jeremy Conlon.

"Prison is a tough place to be, though Paul was a light in the darkness for me and many others," he said.

Prison officials declined to comment.