Media playback is unsupported on your device Media caption June Kelly looks at how Anjem Choudary influenced radicalism in the UK

One of the UK's most notorious radical clerics has been convicted of inviting others to support the so-called Islamic State, it can now be reported.

Police said Anjem Choudary, 49, had stayed "just within the law" for years, but was arrested in 2014 after pledging allegiance to the militant group.

Many people tried for serious terror offences were influenced by his lectures and speeches, police said.

Choudary was convicted alongside confidant Mohammed Mizanur Rahman.

Counter-terrorism chiefs have spent almost 20 years trying to bring Choudary, a father of five, to trial, blaming him, and the proscribed organisations which he helped to run, for radicalising young men and women.

Both men were charged with one offence of inviting support for IS - which is contrary to section 12 of the Terrorism Act 2000 - between 29 June 2014 and 6 March 2015.

The verdict on the two defendants was delivered on 28 July, but can only be reported now, following the conclusion of a separate trial at the Old Bailey of another group of men for a similar offence.

How Anjem Choudary's mouth was finally shut

Anjem Choudary's links to Belgian extremist group

The trial heard how the men decided in the summer of 2014 that the group then known as Isis [Islamic State in Iraq and al-Sham/the Levant] had formed a "Khilafah", or Islamic state, that demanded the obedience and support of Muslims.

'Turning point'

They then invited others to support IS through speeches and announced their own oath of allegiance to its leader.

The oath of allegiance was a "turning point" which meant they could be put on trial, the Met Police said.

Choudary was once the spokesman for al-Muhajiroun, an organisation that can be linked to dozens of terrorism suspects.

Its leader Omar Bakri Muhammad fled the UK after the London suicide bombings on 7 July 2005, and over the years since, Choudary has become one of the most influential radical Islamists in Europe and a string of his followers have either left the UK to fight in Syria or tried to do so.

Media playback is unsupported on your device Media caption Adam Deen explains why Choudary appealed to him

The court heard that in one speech in March 2013 Choudary made it clear that he wanted the Muslim faith to "dominate the whole world".

"Next time when your child is at school and the teacher says 'What do you want when you grow up? What is your ambition?', they should say 'To dominate the whole world by Islam, including Britain - that is my ambition'," he said.

Denounce execution

When IS announced a "Khilafah" - an Islamic state - in June 2014, the court heard that Choudary held a meeting with his closest aides at a curry house in east London.

Before accepting the "Khilafah" was legitimate, the jury heard he consulted his "spiritual guide" Omar Bakri Mohammed, who is currently in jail in Lebanon.

On 7 July 2014, the men's names appeared alongside Rahman's on the oath, which stated the al-Muhajiroun had "affirmed" the legitimacy of the "proclaimed Islamic Caliphate State".

During his trial Choudary also refused to denounce the execution of journalist James Foley by Mohammed Emwazi, nicknamed Jihadi John, in Syria in 2014.

"If you took an objective view there are circumstances where someone could be punished," he told the jury.

Supporters of Choudary included:

Michael Adebolajo and Michael Adebowale, the murderers of soldier Lee Rigby

Suspected IS executioner Siddhartha Dhar

Omar Sharif, a British suicide bomber who attacked Tel Aviv in 2003

Brusthom Ziamani, jailed 12 years later for planning to kill in the streets of London

The trial also heard that Rahman - previously convicted of soliciting to murder - went on Facebook to tell his followers that migration to IS territory was a "duty".

"Let's be clear, the Muslims in the Khilafah need help," he wrote, after communicating with a British fighter who urged him to find recruits.

"The one who is capable to go over and help the Muslims, must go and help."

Image caption Anjem Choudary was convicted alongside his associate Mohammed Mizanur Rahman

Commander Dean Haydon, head of the Met Police's counter-terrorism unit, said the case which led to the conviction of Choudary and Rahman was a "significant prosecution in our fight against terrorism".

He said: "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."

He said the trial had considered over 20 years' worth of material over more than 333 electronic devices containing 12.1 terabytes of storage data.

Sue Hemming, head of counter-terrorism at the Crown Prosecution Service, said the men "knowingly sought to legitimise a terrorist organisation and encouraged others to support it".

"They used the power of social media to attempt to influence those who are susceptible to these types of messages, which might include the young or vulnerable," she added.

Choudary currently has more than 32,000 followers on Twitter and his account can still be viewed online, despite requests for its removal in August last year and the following March.

He and Rahman will be sentenced at the Old Bailey on 6 September.

In a separate trial at the Old Bailey on Tuesday, Mohammed Alamgir, Yousuf Bashir and Rajib Khan, from Luton, were also convicted of encouraging others to support IS.

The three men, who had links with Choudary, gave speeches encouraging people to join the group and not to "sit on the sidelines".

'He liked playing games'

By BBC home affairs correspondent Dominic Casciani

Image copyright PA

The flag of Islam will fly over Downing Street, was his favourite prediction, followed by some kind of rhetorical flourish: "The Muslims are rising to establish the Sharia... Pakistan, Afghanistan and perhaps, my dear Muslims, Londonistan."

He would greet the journalists with a smile, and some guile, dressed up as charm.

He liked playing games. It gave him a sense that he was winning.

Except it wasn't a game. The evidence now shows that Anjem Choudary is one of the most dangerous men in Britain. Not a bomb-maker. Not a facilitator. But an ideologue, a thinker, who encouraged others not to stop and think for themselves before they turned to violence to implement their shared worldview.

How Anjem Choudary's mouth was finally shut