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A new definition of extremism will block certain groups from government funding and meeting officials.
Communities Secretary Michael Gove set out the changes in the House of Commons.
What is the new definition of extremism?
It says extremism is "the promotion or advancement of an ideology based on violence, hatred or intolerance, that aims to:
1. negate or destroy the fundamental rights and freedoms of others; or
2. undermine, overturn or replace the UK's system of liberal parliamentary democracy and democratic rights; or
3. intentionally create a permissive environment for others to achieve the results in (1) or (2)"
The government said it was "narrower and more precise" than the previous definition, which was introduced in 2011.
Which groups might be on the extremism list?
Groups that meet the definition will be blocked from government funding and meeting officials.
Mr Gove listed organisations that he said were a cause for concern under the new definition and would be assessed. They are:
Patriotic Alternative
Anti-extremism campaign group Hope Not Hate describes Patriotic Alternative as the UK's largest fascist group.
Some of its followers were involved in the group National Action, which was subsequently banned under terrorism laws.
Patriotic Alternative says its is promoting a patriotic view of the UK - but its critics say followers regularly espouse Nazi thinking.
British National Socialist Movement
British National Socialist Movement has been described in Parliament as a white supremacist group that originated in 1968.
Muslim Association of Britain
The Muslim Association of Britain (MAB) was described by Michael Gove as the UK affiliate of the Muslim Brotherhood, an international and complex network of Islamist organisations.
Islamism is a political ideology that believes that predominantly Muslim countries should be run according to a form of government drawn from Islam's holy book, the Quran.
Islamism has many branches. Some Islamist groups are so extreme they are also terrorist - but that is not true for every such organisation.
The MAB denies it is an affiliate group and says it has long worked within the law and alongside agencies including the police.
In 2015, then PM David Cameron commissioned a report into the Muslim Brotherhood including any activities in the UK.
Cage
Cage emerged as a campaign group against the "War on Terror" that was launched by President George W Bush in the wake of the 2001 attacks on the US.
It explicitly campaigned against what it said were abuses perpetrated against Muslims in Afghanistan and elsewhere - but its critics say it was often acting for terrorism suspects.
It has never been accused of involvement in terrorism and one of its founders has repeatedly won legal cases where he has been accused of involvement in suspect groups.
MEND
Mend, Muslim Engagement and Development, is a Muslim campaign group that tries to encourage people to get more involved in politics and is active in opposing policies that critics say perpetuate a them-and-us victim mentality.
The organisation says it legitimately campaigns to protect the rights of British Muslims and has said it will take the government to court if it is named as extremist.
- Gove names groups as he unveils extremism definition
Responding to Mr Gove's announcement, MEND said the new definition was a "blatant attack on civil liberties", while the Muslim Association of Britain described it as "blatant effort to stifle dissenting voices".
Cage said it would "challenge the government's deep dive into authoritarianism". Patriotic Alternative said it was a "dangerous step toward a totalitarian state".
The government said there would be a "high bar" to being classed as extremist and the policy would not target those with "private, peaceful beliefs".
Groups and individuals labelled extremist will have the right to seek reassessment and submit new evidence to a review.
Organisations or individuals added to the list will not be criminalised, unlike terrorist groups.
A new Counter-Extremism Centre of Excellence will be launched to gather intelligence and identify extremist groups.
Why is there a new definition of extremism?
Mr Gove told MPs that the UK's values were "under challenge" from extremist groups, which were radicalising young people.
The conflict between Israel and Hamas had led to a "terrible increase" in antisemitic and anti-Muslim hate crimes, Mr Gove added.
Earlier this month, Prime Minister Rishi Sunak warned that democracy was being targeted by extremists and there were "forces here at home trying to tear us apart".
Labour leader Keir Starmer said Mr Sunak was "right to advocate unity and to condemn the unacceptable and intimidatory behaviour".
What have critics said about the extremism definition?
Concerns have been raised that the new definition could deepen divisions.
Zara Mohammed, head of the Muslim Council of Britain, told BBC Newsnight it could lead to the "unfair targeting of Muslim communities".
London Mayor Sadiq Khan told BBC Radio 4's World at One: "I fear it's going to divide communities and send people underground, rather than us having the opportunity to debate with them, educate them and bring them on the right path."
Some Conservative MPs have suggested people like anti-abortion campaigners, or those with gender-critical views, could end up being captured by the new definition.
But Mr Gove said that "we cherish free speech in this country" and that conservative religious beliefs will not be infringed upon, nor environmental protest groups or anti-trans or pro-trans activists.
Others have pointed out that the new definition does not create a criminal offence.
Jonathan Hall KC, the government's independent terrorism legislation reviewer, said there could be unintended consequences. For example, groups not deemed extremist could say they had been "cleared" by the government.
The move to redefine extremism has also been criticised by three Conservative ex-home secretaries, who have warned against using extremism to score political points in a general election year.
UPDATE 15 March: This piece has been updated to reflect that Michael Gove described the MAB as an affiliate of the MB and that they deny any such link.
