Controversial Islamic preacher Anjem Choudary has been convicted of supporting Daesh.
The 49-year-old radical cleric backed the terrorist organisation in a series of talks posted on YouTube, a trial at the Old Bailey heard.
The verdict was delivered on July 28, but for legal reasons can only now be reported.
Choudary, who faces up to 10 years in prison, was convicted alongside his close confidant, Mohammed Mizanur Rahman. A jury took less than three days to unanimously find the pair guilty.
Counter-terrorism police have spent almost two decades trying to bring Choudary to court.
The trial considered evidence gathered during that time from more than 330 electronic devices containing 12.1 terabytes of storage data.
Security sources believe Choudary is linked to 500 British jihadis who have fled the UK to join the terrorist organisation in Syria and Iraq.
Choudary swore the oath of allegiance to the leader of Daesh three days after its leader, Abu Bakr al Baghdadi, declared he had re-established the ancient Islamic Caliphate.
Choudary and Mizanur then delivered a series of lectures using religious passages to justify their support and urged others to follow their example.
Both men talked about the obligation to "make hijrah" - meaning to travel to the Islamic State - the court was told.
The jury heard two years ago the two men decided that Daesh deserved the support and obedience of Muslims. They then pledged their own allegiance through an oath and pledged to encourage others to follow suit.
Commander Dean Haydon, head of the Met Police's counter-terrorism unit said the conviction was a "significant prosecution in our fight against terrorism".
He added: "These men have stayed just within the law for many years, but there is no-one within the counter-terrorism world that has any doubts of the influence that they have had, the hate they have spread and the people that they have encouraged to join terrorist organisations.
"Over and over again we have seen people on trial for the most serious offences who have attended lectures or speeches given by these men.
"The oath of allegiance was a turning point for the police - at last we had the evidence that they had stepped over the line and we could prove they supported Isis."
Choudary and Rahman will be sentenced at the Old Bailey on September 6.
In 2010, Jewish community leaders welcomed the banning of a radical Islamist group which had glorified suicide bombers and preached antisemitism and was led by Choudary.
Then Home Secretary Alan Johnson proscribed Islam4UK under anti-terror legislation and extended the banning order to the group’s use of other names, including Al-Muhajiroun.