On Saturday night, if you were one of the worried Brits scrolling through Twitter to find out what the hell was happening on London Bridge, you might have noticed a certain trend. A user by the name of @leftisthunter, whose Twitter profile is of a Donald Trump Pepe (the frog symbol embraced by the alt right), tweeted: “Another day of cultural enrichment in Londonistan. Weaponised vehicles, a few dead people. Sadiq Khan’s Londonistan #londonattack.” Another user chimed in: “Another great Islamophobia free day in Londonistan at #londonbridge Feels great doesn't it?”

Then the US President jumped in. “At least 7 dead and 48 wounded in terror attack and Mayor of London says there is ‘no reason to be alarmed!’” Donald Trump tweeted on Sunday.

The alt right’s obsession with London goes beyond telling victims of a terrorist attack “I told you so”. According to the spy known as Omar Nasiri, Britain’s capital was nicknamed “Londonistan” in the Nineties, by French intelligence officials who noticed how radicals used the city as a base. However, it was the publication of Londonistan by Melanie Phillips in 2006 that brought the nickname to a wider audience. She argued that policies of multiculturalism and human rights empowered radical Islamists. (Around the same time, Gautam Malkani published Londonstani, described by Good Reads as “a hilarious send-up of multiculturalism”.)

In Londonistan, Phillips wrote:

Britain has become a decadent society, weakened by alarming tendencies towards social and cultural suicide. Turning upon itself, it has progressively attacked or undermined the values, laws and traditions that make it a nation, creating a space that in turn has been exploited by radical Islamism.

Phillips’s book was a best-seller. Now, though, Londonistan (capital of Eurabia) exists primarily as a city on social media. Its news is reported by the right-wing blogs of the United States, UK, France and Hindu nationalists in India. In its modern incarnation, Londonistan is not just a breeding ground for radical Islamists, but a dangerous place – “as dangerous as Mogadishu, Baghdad, and Kabul” according to Twitter user @brianalmon. Islamist vigilantes patrol the neighbourhoods, ready to beat up any transgressors of sharia law. Terrorist attacks are commonplace.

The citizens of Londonistan are mainly conservative Muslims (according to the 2011 census, the actual proportion of any kind of Muslims in London is 12.4 per cent). They dress exclusively in black niqabs, and spend their spare time going to Islamic fashion shows, agitating for UK-wide sharia law, banning alcohol from university campuses and building new mosques.

As for the rest of Londonistan, they are feckless “limp-wristed, latte-sipping leftist liberal intellectuals”, as one Twitter user put it. They have rejected the only thing that can help them - guns. Their one talent is an incredible ability to supersede UK immigration policy, since they have turned the city into a safe haven for refugees despite Britain's lacklustre commitment to resettlement programmes.

In 2016, the citizens of Londonistan did the unforgiveable – they elected the centre-left Labour candidate for Mayor, Sadiq Khan. The Drudge Report, a popular right-wing website, reported the election of Khan as “First Muslim mayor of Londonistan”. Others referred to him as "London's sultan".

One of Khan’s first acts was to ban an advert from the tube which showed an emaciated woman in a bikini with the slogan “Are you beach body ready?” Feminists who had been campaigning against the advert for weeks celebrated, but Londonistan watchers drew a different conclusion.

The blog Bare Naked Islam declared:

Sadiq Khan, the new Muslim Mayor of London, has declared that pictures of scantily-clad female models will be banned from ads on public transportation, ushering in some of the most fundamentalist Islamic censorship policies in the Western world. (Hey, you Londoners voted in a Muslim for mayor, what did you think you were going to get?)

Khan, who voted for same-sex marriage while a Labour MP and describes himself as a “proud feminist”, once famously invited Trump to come to London and meet his family. Nevertheless, in the alternative world of Londonistan, one quote is attributed to him again and again – that terrorist attacks are “part and parcel of life in a big city”.

In fact, the quote comes from a September 2016 interview Khan did with the Evening Standard after an explosion in New York.

He went on to say: "It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things. That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but also it means exchanging ideas and best practice."

Londonistan is, of course, a fiction. After Trump tweeted about Saturday's attack, the Mayor of London’s office said Khan had “more important things to do than respond to Donald Trump’s ill-informed tweet that deliberately takes out of context his remarks”. Its drinking culture continues to flourish - as anyone who has been on a hen night in the capital knows. British Muslims were quick to condemn the terror attacks.

But all this is unlikely to calm the hysteria of Londonistan watchers. They will not see how impoverished students co-exist happily with the all-you-can-eat buffets provided by Muslim restauranteurs, or how mini-skirt-clad and hijab-wearing shoppers rub shoulders on Oxford Street, or how the parks on a summer's day are crammed with all kinds of Londoners, united in embracing a rare day without rain. After all, they will not visit the city itself. It’s too dangerous for that.