A culture of verbal prohibition is taking over society, led by an army of self-appointed militants who see themselves as the guardians of correct thinking

Though it was meant as a joke, it turned out to be a deadly serious affair.

When Professor Tim Hunt, the Nobel Prize-winning biochemist, recently addressed a major conference in Seoul about women in science, he said he had three problems with 'girls in laboratories', namely that: 'You fall in love with them, they fall in love with you, and when you criticise them, they cry.'

As a media storm erupted, Hunt insisted that his remarks were made in 'a totally jocular, ironic way'.

But none of that matters to today's self-righteous crusaders against offensive opinions, for whom language is never a laughing matter. Professor Hunt was immediately surrounded online by an outraged Twitter mob demanding his head on a pike.

These boycott-and-ban zealots are not content with exercising their right to criticise somebody who makes an offensive joke. Quivering with self-righteous indignation, they want to silence those who fail to conform to their group-think.

In this case, their bullying worked. The scientific authorities immediately caved in. Not only was Hunt forced to resign from his post at University College London, but he was also dismissed from the science committee of the European Research Council.

So biochemistry loses a brilliant pioneer, while we lose more ground to the forces of oppressive censorship.

For the saga of Tim Hunt's downfall is just the latest example of how a culture of verbal prohibition is taking over society, led by an army of self-appointed militants who see themselves as the guardians of correct thinking.

Desperate to find offence at every turn, eager to suppress any utterance that deviates from their conformist views, these are the warriors of the 'You Can't Say That' brigade.

The great 19th-century liberal philosopher John Stuart Mill warned against 'the tyranny of prevailing opinion', but that is exactly what we are now sliding towards in early 21st-century Britain.

Professor Hunt was immediately surrounded online by an outraged Twitter mob demanding his head on a pike

As a media storm erupted, Hunt insisted that his remarks were made in 'a totally jocular, ironic way'

The intimidators are all around us, endlessly peddling their permanent outrage and demands for punishment. Only last week, a petition was launched on Twitter calling for TV presenter Kay Burley to be sacked from Sky News, on the grounds that she had interviewed a company executive too robustly.

Absurdly, the petition quickly gathered more than 55,000 supporters. Only a couple of months earlier, Elton John called for a boycott of the fashion brand Dolce & Gabbana because the two Italian designers criticised gay parenthood and the use of fertility treatment.

This relentless indignation over language is causing profound damage, not just to individuals, but also to the very concept of free speech, which is one of the pillars of our civilisation.

The essence of humanity is to think freely and rationally, to challenge conventional wisdom and to encourage new ideas. As George Orwell put it: 'If liberty means anything at all, it means the right to tell people what they don't want to hear.'

Over centuries, freedom of speech has been the single most important factor in driving scientific progress and the advance of democracy.

That is precisely why powerful vested interests, such as despotic regimes and wealthy elites, have always fought so hard to restrict it. They have known that the surest way of maintaining their authority is to close down debate and prevent free expression.

That is why the 18th-century Parliament, dominated by the aristocracy, was so ferociously opposed to the publication of any reports about its proceedings, and why the Churches used to be so determined to root out heretics who dared to challenge their doctrines.

But the heresy-seekers and witch-finders of today's Britain are very different from the ultra-conservative rulers and religious bigots of the past.

Only last week, a petition was launched on Twitter calling for TV presenter Kay Burley to be sacked from Sky News, on the grounds that she had interviewed a company executive too robustly

The 'You Can't Say That' merchants — who count disproportionate numbers of academics, intellectuals, writers and artists among their number — like to see themselves as liberal-minded and progressive.

In a bitter irony, they claim it is their supposed compassionate liberalism that drives their enthusiasm for the repression of free speech.

Their 'intolerance of intolerance', to use one of their paradoxical mantras, makes them want to banish offensive language and protect the vulnerable from dangerous opinions.

They never openly admit that they favour censorship, yet their eagerness to use the 'but' word gives them away. So you constantly hear them proclaim their belief in free expression, only to heavily qualify that support.

A classic example was the statement from Mary-Kay Wilmers, the editor of the London Review of Books, after the Islamist massacre of cartoonists at the offices of the French satirical magazine Charlie Hebdo in January. 'I believe in the right not to be killed for something I say, but I don't believe that I have the right to insult whomever I please.'

Those weasel words amount to a surrender to the enemies of liberty. As Baruch Spinoza, the great Dutch philosopher and pioneer of the enlightenment, put it 350 years ago when challenging religious dogma: 'In a free state, every man may think what he likes and say what he thinks.'

Elton John called for a boycott of the fashion brand Dolce & Gabbana because the two Italian designers criticised gay parenthood and the use of fertility treatment

But that is certainly not the way the 'You Can't Say That' lobby see it. Freedom of speech used to be regarded by progressives as a weapon of change and a means to give a voice to minorities such as gays and black people. Today, the very opposite is true.

The same freedom is now viewed as something threatening. Like the kings and cardinals of the past, the modern guardians of morality believe that language must be controlled in order to prevent the spread of heresy, dissent, anarchy and conflict. Such a restrictive approach reflects a deeply patronising view of the public, portraying us as so gullible that we can easily be inflamed by a few unsuitable words.

The saga of Tim Hunt's downfall is just the latest example of how a culture of verbal prohibition is taking over society, led by an army of self-appointed militants who see themselves as the guardians of correct thinking

It is as if the crusaders for repression think that beneath the surface of society there lurks a cauldron of bubbling hatred, which can only be held in check by rigorously enforced speech codes.

That is the thinking that led to the Leveson inquiry, and the demands to shackle the Press through a State-backed regulator, something that has not existed in Britain since 1695.

In the febrile atmosphere created around the phone-hacking scandal by pressure groups such as Hacked Off, there was a mood that the rambunctious tabloids had to be sanitised. This sustained attack on the Press gave rise to some surreal moments, like the sight of former Monty Python stars — once renowned for their irreverent willingness to poke fun at the Establishment — solemnly lining up to demand State-backed control of the Press.

John Cleese even compared journalists to 'murderers' at one Hacked Off rally, claiming that, like the Press, 'they would make a very good case' for self-regulation.

Attacks on the Press are not the only weapon in the arsenal of the State. In recent years, the Government has acquired a host of new powers to crack down on free speech, including the 2003 Communications Act, and the 2006 Racial and Religious Hatred Act, which makes it an offence to use 'threatening words or behaviour' to stir up hatred.

It is estimated that more people are now jailed or arrested in Britain for what they think, believe or say than at any time since the 18th century.

Under the 1986 Public Order Act, there are 25,000 criminal proceedings on average each year for speech offences, half of which result in convictions.

The entire civic atmosphere is one of institutionalised vigilance against any language deemed to be offensive.

In January, in the wake of the Charlie Hebdo killings, world leaders gathered in Paris to proclaim their determination to defend free speech. But they never meant it

Leading black American lawyer David Baugh defended a racist Ku Klux Klan leader in 1999 who had been charged with burning crosses to intimidate black people at a rally in Virginia.

Assuring the jury that he was aware of the Klan's visceral hostility to black people such as him, Baugh said: 'In America, we have the right to hate. And we have the right to discuss it.'

Such a statement would be unthinkable in our society today, where so-called hate speech has been made a serious crime. But offensive language is not merely deemed to be hateful, it is also held to be psychotic, as if the speaker is suffering from a kind of mental illness, hence the use of terms like homophobic and Islamophobic.

It is estimated that more people are now jailed or arrested in Britain for what they think, believe or say than at any time since the 18th century

One of the more fashionable labels is now 'transphobia', which is used to close down any debate about issues raised by gender identity changes. Transgender activists have even succeeded in getting some feminists, such as the lesbian and gay rights campaigner Julie Bindel, banned from speaking at events because of their alleged 'transphobic' attitudes.

Closely related to this nonsense is the accusation of being a 'denier'. It is a charge used to show that the speaker's opinions are so outlandish and offensive that they must be sidelined or suppressed.

The term originated with the drive to prevent Right-wing extremists from denying the existence of the Holocaust against the Jews, but is now used indiscriminately to hound those with controversial opinions.

In 2014, a group of UK-based environmental campaigners argued that 'climate change deniers' are 'responsible for crimes against humanity' and should face 'Nuremberg-style trials' for 'actively spreading doubt' about the orthodoxy on global warming.

Perhaps the most famous words on free speech were ascribed to the 18th-century French writer Voltaire, who, according to his biographer, said: 'I disapprove of what you say, but I will fight to the death for your right to say it.'

Yet we now live in an age of what I call 'Reverse Voltaires', whose attitude is: 'I know I will detest what you say, and I will defend to the end of free speech my right to stop you saying it.'

These people make their influence felt right across our society, from science labs to football stadiums, from the Twittersphere to comedy clubs.

Propped up by State intervention and the worsening climate of self-censorship which is a result of so much bullying, they are waging a war on free speech — in the name of protecting us from alleged off ensiveness.

This is at its worst in our universities, which should be bastions of freedom but instead are being turned into dreary citadels of conformity.

In the Seventies, the National Union of Students campaigned to ban extremism from campuses with the cry of 'no platform for racists and fascists'. Today that idea has been transmuted into a ban on almost anything controversial, on the grounds that students are too delicate to face anything uncomfortable.

These boycott-and-ban zealots are not content with exercising their right to criticise somebody who makes an offensive joke. Quivering with self-righteous indignation, they want to silence those who fail to conform to their group-thin

It was in this spirit that last year the University of East Anglia politics society banned a visit by a UKIP local candidate. 'This is about ensuring that UEA students are on a campus where they feel safe, secure and respected,' said a representative of the society, defending the rejection of someone whose party has just polled nearly four million votes in the General Election.

It is the same mentality that has led to the growing fashion for university students to demand that course material deploys so-called 'Trigger Warnings'. These are labels at the front of a book or film that warn that the work may involve offensive words or images.

Fear of anything that strays from what is perceived to be acceptable is partly driven by modern identity politics, which promotes competitive victimhood by giving status and influence to those who can assert their experience of oppression.

The suffocating blanket of caution and censorship is not just weakening the robustness of the public. It is also destroying the robustness of debate and free expression, on which democratic society depends.

In January, in the wake of the Charlie Hebdo killings, world leaders gathered in Paris to proclaim their determination to defend free speech. But they never meant it.

Worse, their governments were cracking down on offensive speech in practice while apparently defending freedom in principle. If they really wanted free speech to flourish, they would stop the 'You Can't Say That' brigade.

You can't just defend the parts of free speech that meet your own standards and prejudices — otherwise it becomes a privilege rather than a right, and you might as well not have free speech at all. And that, as history has taught us, sounds the death knell for a free and democratic society.

Trigger Warning: Is The Fear Of Being Offensive Killing Free Speech? by Mick Hume is published by HarperCollins at £12.99.