A convicted Isis supporter who knew the Westminster attacker has admitted disseminating terrorist propaganda after his release from jail.
He also broke notification requirements for convicted terrorists, and downloaded Isis propaganda magazines containing guides on launching terror attacks.
On Wednesday, Anderson admitted committing a total of 15 terror offences after being freed from prison.
The case comes amid mounting concerns over the impact of jail sentences on terror offenders, radicalisation inside prisons and the effectiveness of deradicalisation programmes.
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Anderson was sentenced to three years imprisonment in 2016 for promoting Isis outside Topshop in London’s Oxford Street.
It later emerged that he went to the same Luton gym as the Westminster attacker, Khalid Masood, and an Isis fighter who was killed in a drone strike.
Anderson, now 44, was one of numerous terror convicts linked to Anjem Choudary’s banned al-Muhajiroun network.
Following his release from prison, he was subjected to terrorist notification requirements, which are intended to enable police to monitor convicts and manage any risk they pose.
But he was able to commit a string of terror offences through summer and autumn last year.
Appearing at London’s Old Bailey on Wednesday, Anderson pleaded guilty to 10 counts of disseminating terrorist publications on Facebook and Telegram in July.
They included official Isis propaganda films showing execution and beheadings, calling for terror attacks and glorifying suicide bombings.
The defendant, who entered his pleas via a video-link from HMP Wandsworth, admitted a further four charges of possessing terrorist publications. They related to issues of Isis’ official Rumiyah magazine, including editions containing instructions on knife and vehicle attacks.
Anderson also pleaded guilty to breaching terrorist notification requirements by not giving police details of current email accounts.
Patrick Harte, for the defence, told the Old Bailey that Anderson admitted dissemination “on a reckless basis in the sense he did not intend his act to encourage terrorism”.
Mr Harte told the court his client said he must have inadvertently downloaded the other documents when he downloaded the films. He said Anderson did not open them or forward them to anyone else.
Judge Philip Katz QC ordered a pre-sentence report and remanded Anderson in custody until a further hearing on 30 April.
Concerns had been publicly raised about Anderson’s online activity before the period covered by the case.
In 2019, The Sunreported that he had shared hateful images on his Facebook page including one showing an AK-47 assault rifle laid in front of a prayer mat.
The caption said: “Remember the Mujahideen [warriors] in your prayers because they are fighting on your behalf.”
Anderson lived in Luton, where he attended a gym that was frequented by other extremists.
Inquests into the March 2017 Westminster attack, which left five victims dead, heard he knew terrorist Khalid Masood through the gym, as well as a man called Abu Rahin Aziz who was later killed while fighting for Isis in Syria.
Anderson, a former mechanic with five children, has previous convictions for assault and possession of an imitation firearm.
Born as Andrew Anderson, he converted to Islam while serving a jail sentence for robbery in the 1990s.
Before his 2016 prison sentence, he was known for preaching from stalls linked to the al-Muhajiroun extremist network.
In August 2014, he was spotted on Oxford Street handing out a leaflet on the Isis “caliphate” and challenged by two Muslim women who reported him to police.
Photos of the leaflets circulated on social media showed them declaring the “dawn of a new era” and calling on Muslims to pledge allegiance and migrate to the new so-called Islamic State.
The case resulted in his prosecution for inviting support for Isis, which was the same offence used against al-Muhajiroun leaders Anjem Choudary and Mizanur Rahman.
Despite being hailed as the UK’s “most dangerous extremist group” and banned as a terrorist organisation since 2006, no one has ever been successfully prosecuted for membership of al-Muhajiroun itself.
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