Media watchdog Ofcom is investigating UK channel Peace TV for giving a platform to banned Muslim preacher Zakir Naik.
Dr Naik was banned from entering Britain last year by Home Secretary Theresa May after allegedly claiming America was "controlled by the Jews".
An Ofcom spokesman confirmed: "We are in the middle of an investigation about Peace TV.
"Ofcom will not tolerate extremism on British television, and transgressors will be dealt with."
The channel also hosts controversial preacher, Dr Abu Ameenah Bilal Philips, who was banned from entering the UK last June and Sheihk Abdullah Hakim Quick, who has previously called Jews "filth".
Mark Gardner, of the Community Security Trust, said: ""Peace TV features a number of problematic preachers and appears to be yet another Islamist propaganda media outlet."