Anjem Choudary has been jailed for five-and-a-half years for inviting support for ISIS.
The 49-year-old was convicted at the Old Bailey after backing the group in an oath of allegiance published online. Police say Choudary’s followers have carried attacks in the UK and abroad.
The judge, who described Choudary as calculating and dangerous, passed the same sentence on his follower Mohammed Mizanur Rahman, 33.
The trial heard the pair also used speeches to urge support for ISIS after it declared a Caliphate in the summer of 2014.
Choudary and Rahman were convicted last month of inviting support for ISIS – an offence contrary to section 12 of the Terrorism Act 2000 – between 29 June 2014 and 6 March 2015.
Counter-terrorism chiefs blame the preacher and the proscribed organisations which he helped to run, such as al-Muhajiroun, for radicalising young men and women including the killers of soldier Lee Rigby in 2013.
But they said they had been unable to act for many years as he had stayed “just within the law”.
Passing sentence, the judge, Mr Justice Holroyde, said the evidence showed very clearly that the defendents held views that were strongly supportive of ISIS.
He told them: “You are free to hold your views, but Parliament has made it an offence to invite support for a proscribed organisation.”
He said the jury were sure that they knowingly crossed the line and that “each of you gave a published oath of allegiance to the ISIS leader and gave lectures in 2014.”
Mr Justice Holroyde said Choudary’s influence could be seen in how his followers revered him in social media messages and that he “indirectly” contributed to terrorism through his words and their effects on others.
He said the timing of the communication, his high standing and large audience of “impressionable persons” equalled an indirect link to terrorism that was serious.