Radical cleric Anjem Choudary, long a thorn in the side of British authorities, was released from prison on Friday having served half his sentence for encouraging support for the militant Islamic State (IS) group, British media reported.

The 51-year-old was jailed for five-and-a-half years in 2016, and will serve the rest of his sentence under strict supervision orders having been released from Belmarsh prison, near London.

He is expected to return to his home in Ilford, east of London, although will not be able to use any internet-enabled devices without permission, the BBC reported.

Other restrictions are reported to include bans on leaving Britain without permission and on attending certain mosques and he will only be allowed to meet with people approved by the police.

Choudary is the former head in Britain of Islam4UK or al-Muhajiroun, a now-banned group co-founded by Omar Bakri Muhammad that called for Islamic law in the United Kingdom.

For two decades, the former lawyer of Pakistani descent stayed on the right side of the law, becoming Britain's most prominent radical preacher.

Among those radicalised by Muhajiroun were the suicide bombers who killed 52 people on London's public transport system in July 2005, and the men who murdered soldier Lee Rigby in the capital in 2013, police say.

The court heard that Choudary had broadcast speeches recognising Abu Bakr al-Baghdadi as the leader of the IS.

Choudary and his co-defendant Mohammed Rahman were arrested by officers from the Metropolitan Police's Counter Terrorism Command on September 25, 2014.

The father-of-five previously hit the headlines for organising a pro-Osama bin Laden event in London in 2011.

He also belonged to a group that burned poppies, the symbol of remembrance for deaths in war, during an Armistice Day protest in the British capital in 2010.

In a 2014 interview with AFP, Choudary called on western journalists, civilians and troops in “Muslim countries” to “completely withdraw and allow us to implement the Sharia”.

Former police terror chief Richard Walton called him a “hardened dangerous terrorist” who had had a “huge influence on Islamist extremism in this country”.