LONDON — One of Britain’s best-known Islamist activists has been found guilty of inviting support for the Islamic State and could face a prison term of up to 10 years, officials announced on Tuesday.

The activist, Anjem Choudary, 49, and an associate, Mohammed Mizanur Rahman, 33, were convicted of using online lectures and messages to encourage support for the Islamic State, also known as ISIS or ISIL, which is banned in Britain.

In social media posts, the two men pledged allegiance to the caliphate declared by the head of the Islamic State, Abu Bakr al-Baghdadi, and said that Muslims had a duty to obey and support him.

The men were arrested in 2014 and were tried at the Old Bailey in London, the central criminal court. The jury delivered its verdict on July 28, but it was not announced until Tuesday for legal reasons. Sentencing is scheduled for Sept. 6.