What are the hurdles facing Michael Gove's extremism plans?

Michael Gove once made the front page of the Scottish Sun newspaper for being pictured at a nightclub at 02:30 in the morning.

A related image comes to mind when trying to sum up what the communities secretary is trying to achieve with his redefinition of extremism in British public life.

Think of him as the nightclub bouncer: Mr Gove's announcement on Thursday aims to establish a strict door policy for who gets access to government. If your name is on that list, you're not getting in. Anyone labelled as an extremist will be barred from meeting ministers, helping to shape public policy or seeking public funds.

Mr Gove told MPs he was driven to action because the UK's values were "under challenge" from extremist groups, which were radicalising young people.

But this is not a new plan.

For 20 years, a succession of governments have tried to define and, at times, ban extremism. Tony Blair was the first, after the 2005 terrorist bombings of London. David Cameron twice promised an extremism ban.

Each time, the government has faced the same intellectual and legal problem: nobody agrees what the word means.

Terrorism is very clearly defined in British law as the use or threat of violence to intimidate government or society to follow a particular course in the name of an ideological cause.

But extremism is not a crime. And one politician's extremism is another activist's legitimate cause. Think of the Suffragettes a century ago (although some were involved in a bombing campaign) or radical climate change protesters blocking roads today.

"Most extremist materials and activities are not illegal and do not meet the terrorism or the national security threshold," Mr Gove admitted in his Commons speech.

The most we have had as an official working definition of extremism dates from 2011, when David Cameron's government declared it meant, among other things, "vocal or active opposition to fundamental British values".

What's a fundamental British value? Does everyone even know what they might be, never mind agree on them? This is the problem that policymakers and government lawyers have struggled with.

Mr Cameron had to abandon a proposal in 2015 to ban the Muslim Brotherhood, the most important international Islamist network - when officials looked for the evidence linking the Brotherhood and its British offshoots to violence they didn't find it. And so it could not be banned under terrorism laws.

Nine years on, Mr Gove's argument is that his new definition of extremism is more workable than the 2011 version because it doesn't criminalise groups involved in activities he says are driving division in society, but rather allows ministers and policy-makers to say they won't engage with them.

He listed five groups he said were cause for concern, including a small far-right group that calls for the expulsion of ethnic minorities and Islamist organisations that he says want to tear down democracy.

It's not clear what his list will really lead to - other than accusations thrown back at him that he is driving the wedge.

For a start, none of the groups he has so far named are at the centre of policy-making. As no new crime is being created, police will be under no obligation to round up suspected extremists.

For weeks the government has been lobbied to use new extremism powers to stop the Gaza protests. But Mr Gove's plan has no effect on the right to free speech and protest. Demonstrations can only be banned if police chiefs have credible evidence they are likely to lead to uncontainable disorder.

Other public bodies are under no obligation to adopt the definition - not least universities.

This brings us to one of the first potentially unintended consequences.

Last year, the government placed a new legal duty on universities to protect free speech. Mr Gove could today label a group as extremist but under this plan, be unable to stop its members promoting their views to students on campus.

Jonathan Hall KC, the independent reviewer of terrorism laws, has identified other problems.

"A bank might decide we're going to get rid of you as a customer, because we don't want to have extremists on our books," he says.

A genuinely hateful person who is a foreign national, and who crosses the threshold for ejection from the UK, might also be able to use the label to their advantage, warns Mr Hall. They might try to challenge their removal to a despotic regime saying their naming-and-shaming in the UK means they could be tortured back home - something our law absolutely forbids.

"What happens if a group is not labelled?" he asks. "Does that mean that that group, even though they may have some quite unacceptable ideas, can run around saying [they've'] been cleared by the government?"

Mr Gove says his aim is to protect democracy, but even some in his own party are unconvinced this will work.

MP Miriam Cates has warned it could have a "chilling effect" on gender-critical feminists who want to reverse law that allows people to change sex on their birth certificate.

Mr Gove says "gender-critical campaigners, those with conservative religious beliefs, trans activists, environmental protest groups, or those exercising their proper right to free speech" are not his target.

And it is true that he has long campaigned, as a journalist and a politician, for what he regards as a tougher response to the threat of terrorism.

His views that some lawful Islamist groups may pose a threat to society are hotly contested, but he is not alone in his thinking.

It is a racing certainty that now the policy has been made real it will be challenged in the courts. One of the Muslim groups he named on Thursday, MEND, is already preparing a case.

But it could take nine months or longer for any group that's labelled extremist to get a judgment that forces government to rethink the plan.

That would take it beyond the general election, and if the polls are correct, it would no longer be Mr Gove's problem.

But his critics will say he benefitted from pre-election headlines in a policy area that has been contested for 20 years.