The failure to effectively tackle extremism is creating an "ever-bigger pool" from which terrorist groups can recruit, an official watchdog has warned.
The Commission for Countering Extremism said the "gaping chasm" in existing legislation means many groups – from radical Islamists to far-right neo-Nazis – are able to operate with impunity.
It called on ministers to outlaw the "praising and glorifying" of terrorists and their actions as part of a new legal framework for dealing with the issue.
The findings were backed by faith leaders, including the Archbishop of Canterbury, the Chief Rabbi and the chair of Mosques and Imams National Advisory Board, human rights campaigner Peter Tatchell and former prime ministers Tony Blair and David Cameron.
The commission – which was established in the wake of the 2017 London Bridge attacks – said current legislation was focused on dealing with the threat of terrorism.
However, it meant that much extremist activity – so long as it did not cross a certain threshold – was not covered by the law.
It highlighted the case of the hate preacher Anjem Choudary, who was thought to have motivated between 70 and 100 people to turn to terrorism over a number of years.
However, he was able to operate lawfully and freely in Britain until he was eventually convicted of the specific terror offence of inviting support for a proscribed organisation – so-called Islamic State (IS).
In its report, the commission said it had found online extremist messaging boards which glorified figures such as Osama bin Laden and the 9/11 hijackers as well as far-right terrorists such as Anders Breivik, Brenton Tarrant and Thomas Mair.
At the same time, the law meant that collecting IS beheading videos or forming neo-Nazi groups which praised Adolf Hitler and encouraged Holocaust denial was not illegal as long as it was not threatening, abusive or insulting.
Former Assistant Commissioner Sir Mark Rowley, the ex-national police lead on counter-terrorism who helped draw up the commission's report, said the scale and nature of the material that was freely available was "quite extraordinary".
"During the course of conducting this review, I have been shocked and horrified by the ghastliness and volume of hateful extremist materials and behaviour which is lawful in Britain," he said.
"Not only have our laws failed to keep pace with the evolving threat of modern-day extremism, current legal boundaries allow extremists to operate with impunity.
"Hateful extremism is creating an ever-bigger pool for terrorists to recruit from, as well as increasing violence, hate crime and tensions between and within communities. The current situation is simply untenable."
The report said past attempts to capture "hateful extremism" in law through the "lens of counter terrorism policy" had proved "futile and flawed".
It noted that in 2015, a promised extremism Bill failed to materialise because it was unable to produce a legally acceptable definition of extremism.
While it welcomed proposals in the Government's online harms white paper for a strong regulatory regime, it said that it needed to go "much further" in addressing the issue of extremism.
"We are particularly concerned by the targeted radicalisation of young people and the lack of criminal sanctions against those who intend to radicalise young people into extremism," it said.
"This is despite it creating a climate conducive to terrorism, hate crime, or other violence and/or is attempting to erode and destroy the fundamental rights and freedoms of our democratic society."
Cover image: Library photo of the Houses of Parliament (Picture: PA).