LONDON — Anjem Choudary, one of Britain’s best known Islamist activists, was sentenced on Tuesday to five years and six months in prison after having been convicted in July of encouraging support for the Islamic State.

Mr. Choudary, 49, had been found guilty of promoting the Islamic State, also known as ISIS or ISIL, in speeches and messages posted online, which is a crime under Britain’s antiterrorism laws.

Mohammed Mizanur Rahman, 33, an associate of Mr. Choudary’s, received the same sentence. Along with Mr. Choudary, Mr. Rahman was convicted on July 28 of having violated British laws after a four-week trial at the Old Bailey, London’s central criminal court. The verdict was not announced until mid-August for legal reasons.

Mr. Choudary’s supporters shouted the Arabic phrase “Allahu akbar,” or “God is great,” as he was sent to jail in London.