London is set to play host to one of the most dangerous politicians on the planet this week. Not that you’ll hear any such thing when Narendra Modi arrives. Instead, we’ll be reminded that India’s prime minister is the leader of a giant and dynamic economy. That he’s taking tea with the Queen and buddying up to David Cameron. There’ll be fun Modi facts too: how he once sold chai at railway stations; how, aged 65, he boasts of having a 56-inch chest.

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How can someone so Technicolor be so dangerous? Well, imagine any national leader – Cameron, Merkel, Obama – spending a large chunk of his or her life working for a gang of religious fascists – one that renowned academics compare to Islamic State. Chuck in a long personal history of inciting religious hostility, a track record of cosying up to big business, and a reputation for ruthlessness towards enemies. Now put this extremist in charge of a nuclear state. Worried yet?

That, in a nutshell, is the man who will be jetting into Britain. As a boy Modi joined the far-right Rashtriya Swayamsevak Sangh (RSS), whose objective is to turn India – which gave the world Jainism and Buddhism and Sikhism, and which has the world’s third-largest Muslim population – into a Hindu superpower. Among its alumni is Nathuram Godse, the fanatic who gunned down Mahatma Gandhi.

Religious extremism is not some long-faded part of Modi’s past. In 2002, while he was chief minister for Gujarat, a train carriage carrying Hindu pilgrims caught fire in the state. Within hours, without a scrap of evidence, Modi blamed the 58 deaths on the Pakistani secret services, then paraded the charred corpses through Ahmedabad.

His Bharatiya Janata party (BJP) called a three-day strike. There then followed one of the bloodiest anti-Muslim pogroms in modern history. Mobs of men dragged wives and daughters on to the streets to be raped. One ringleader later boasted of slitting open the womb of a pregnant woman. Between 1,000 and 2,000 people were killed – the vast majority Muslim.

Try as they might, BJP supporters cannot erase the history of these shameful killings or absolve their leader of responsibility. This version of events is not contested by any serious analyst – and at the very least it shows up Modi as a master of hate speech. Asked three years ago whether he felt any regret over the deaths of so many innocent people, the BJP leader replied that he felt the same pain as a passenger in a car that has just run over a puppy.

But this is all about to be consigned to the past. For years after the massacres Britain shunned Modi. But this week it will roll out the red carpet, even as the atmosphere of thuggish intolerance and violence around Modi grows thicker.

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In September he took his cabinet to meet RSS leaders for a three-day summit, where ministers reported on their progress. The RSS has been having meetings with the education ministry to gain greater influence over the curriculum. In Modi’s home state of Gujarat, schoolchildren are already given textbooks written by RSS affiliates.

Primary and secondary pupils are taught that, while television “was invented by a priest from Scotland called John Logie Baird”, it was actually pioneered thousands of years ago, by Hindu royalty in ancient India. So, for that matter, was the motor car. And so was stem-cell research. These textbooks carry praising endorsements from Modi himself. It is as if the dad off Goodness Gracious Me – who claimed everything was invented in India – has been put in charge of an entire nation’s syllabus.

The sad oddity of all this is that India can be genuinely proud of its traditional hospitality towards dissent. A subcontinent of a billion people, of glaciers and deserts, is naturally pluralistic. “There is not a thought that is being thought in the west or the east that is not active in some Indian mind,” wrote the historian EP Thompson.

Yet Hindu extremists now force major publishers to pulp books they deem offensive. Campaign groups such as the Ford Foundation and Amnesty, whose work on human rights and the environment needle Modi’s officials, are put under so much scrutiny that they can barely continue. An environmentalist invited by British MPs to testify on abuse by mining firms was yanked from her London flight just before take-off. And last Friday the Indian arm of Greenpeace was ordered by the authorities to shut down, on the flimsiest of pretexts.

Just as with the Gujarat pogrom, the prime minister has no direct part to play in any of this – rather he fosters the environment that makes it all possible. One incident from this September is typical. A Muslim villager is accused by a Hindu mob of eating beef and lynched. The issue of beef slaughter is one that Modi campaigned on before his election. Now he keeps shtoom – even while his party colleagues issue justifications. Finally, an interview is given in which Modi voices the most watery regret.

By his rise to power, by his strategic silences, by his smirking apologies, Modi gives succour to the gathering mob. He was voted in on a ticket of reviving a moribund economy. Supporters pointed to the apparent success story of Gujarat. They didn’t read the auditors’ reports that showed how the development success of Gujarat lay in giving more money to the urban rich, in handing land and soft loans to the business houses.

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Now that Modi is failing to turn around India, he and his generals fall back on the old trick of hunting for an enemy: Pakistan, religious minorities, pseudo-seculars. An environment now exists in which scholars who criticise Hindu idol worship receive death threats, and are then murdered. An intellectual who invites a former Pakistani minister to give a talk in Mumbai is nabbed by Hindu zealots and smeared with ink. Writers, academics and scientists return their national honours to Delhi in protest at the officially sponsored thuggishness.

Cash-strapped Cameron will never raise these issues with his guest. The permanent secretary at the Foreign Office admitted to MPs just a few weeks ago that human rights no longer count as a “top priority”, and come below the government’s “prosperity agenda”.

Meanwhile, India’s new leader hugs Mark Zuckerberg; he’ll play to the proud Indian diaspora at Wembley Stadium this week; and rules with a giant mandate and an opposition in disarray. “This is the most dangerous leader India has had in 30 years,” says one of the country’s most acute observers, Mihir Sharma. “He reminds me of Putin: appealing to a glorious past, friend to the oligarchs and to a state religion, clamping down on dissent.”

This is what real danger looks like nowadays: wearing a business suit and clutching trade deals – while silencing those who disagree.

• This article was amended on 25 November 2015 because an earlier version incorrectly referred to Ahmedabad as the state capital of Gujarat.