On Saturday, the Sun published an exclusive story by its political editor, Tom Newton Dunn, which announced that a group of former British intelligence officers had uncovered a “hard-left extremist network” at the heart of the Labour party. “HIJACKED LABOUR” declared the piece, which went on to claim that Jeremy Corbyn sits at the centre of a “spider’s web of extensive contacts” that stretch “from Marxist intellectuals to militant groups and illegal terror organisations”.

A global online ecosystem has grown up that bridges the gap between the far right and the mainstream

The piece directed readers towards a website featuring a network map that it said had been compiled by ex-military veterans “in their spare time” to reveal “what they insist is now a party firmly in the grip of a hardline cabal”. Each of the 490 organisations or individuals listed was presented as a node on a network, with an attached fact file and further reading links.

But when readers began to inspect the map more closely, we found that several entries on the chart included extreme rightwing material among their sources. One fact file recommended a “critique” of anti-fascism posted on the antisemitic conspiracy website the Millennium Report – which also features articles on such topics as “the Jewish hand in the world wars” and “exposing Jewish Zionism”. Another fact file pointed readers towards the website Aryan Unity – once the mouthpiece of the British People’s party, a defunct neo-Nazi group. These were presented without caveats, as apparently trustworthy sources.

Before the end of the day, the story had been removed from the paper’s website – without acknowledgement or explanation from Newton Dunn or his bosses. (The Sun declined to comment, and Newton Dunn has not responded to my questions.)

This was more than a case of sloppy sourcing: the map itself is drenched in conspiratorial thinking. It describes obvious political connections – for instance, that Corbyn and other senior Labour MPs are linked to trade union officials – in a way that implies these are part of a wider shadowy network. This network apparently includes the IRA, Hamas and the Colombian guerilla group Farc; “radical” members of the British Medical Association; and various anti-war, anti-racist and anarchist groups, many of whom don’t even get on with each other. The chart also links Corbyn’s Labour party to a range of living and deceased philosophers, including Stanley Fish, “Jacques Derrida RIP” and “Richard Rorty RIP”.

The argument here is that this “hard-left extremist network” shares an insidious and unpatriotic ideology: “postmodern neo-Marxism”. As one of the researchers told the Sun, this is “an ideology which specifically encourages its supporters to tell lies, because it denies the existence of any absolute truth”. Various independent media outlets were featured on the map, including openDemocracy – George Soros was listed among its funders – as well as a number of high-profile journalists, including Guardian contributors.

So how did the Sun, one of the UK’s most widely read newspapers, come to publish it? The answer reveals how easily far-right conspiracy theories now circulate between fringe outlets and the mainstream media. The map linked to by the Sun closely resembles an earlier graphic that first appeared online in August, under the name the Traitor’s Chart. The earliest mention of it that I can find is on an obscure website called Country Squire Magazine, which said it had received a press release about the map. That article, since deleted, had a publication date of 21 August. The website’s only other significant intervention to date has been a pro-Brexit polemic that described Ireland as a “land of puppy farms, rain-soaked holidays, dingy bars, drugs mule celebs, verbal diarrhoea and squeaky fiddles”, also now deleted.

The Traitors’ Chart, which was larger than its successor and included the names of even more high-profile journalists, was then picked up by the international far-right media network, Breitbart, whose UK editor, James Delingpole, wrote about it on 27 August. For Delingpole, the map revealed the extent of “cultural Marxism”, which has led to “the takeover by the hard left of every institution so that even when there’s a supposedly ‘conservative’ government in power, it’s the values of the left which continue to dominate everyday life”.

Since then, the Traitors’ Chart has been revised and rebranded as Hijacked Labour: the website that hosted the original was closed down and a new one registered on 3 December, a few days before the Sun’s “exclusive” appeared, this time with a new frontman for the project – a former SAS officer and author named Mark Bles. (Since the Sun took down its story, the map has been revised once more, removing the links to neo-Nazi and antisemitic websites.)

Conspiracy theories offer a simple way to explain complex power relationships and patterns of cause and effect. Individuals are more likely to turn to them when they feel powerless or when they lack the tools to analyse the world in other ways. They can flourish within any political tradition – look, for instance, at the “Corbyn did Brexit” rhetoric among some liberal remainers, who believe he sabotaged the referendum. The defensive and evasive behaviour of the Labour leadership over antisemitism in its own ranks, meanwhile, has caused great damage. But conspiracy theories become a lot more dangerous when they are cultivated and amplified by prominent media platforms – and this is overwhelmingly taking place on the right.

As the story of the Traitors’ Chart indicates, a global online ecosystem has grown up in the last few years that bridges the gap between the far right and the mainstream – and, in the case of outlets such as Breitbart, there is significant financial backing involved. In the UK, there has been a concerted effort from mainstream conservatives to depict Corbyn, and by extension the whole Labour party, as a deadly threat to the nation. His past associations with Palestinian activists and Irish nationalists; his opposition to British overseas military action – all these and more can be criticised or defended on their own merits, but in the right’s narrative they make Labour not only objectionable but democratically illegitimate.

The election has raised this to a fever pitch – and it comes after three years in which the pro-Brexit right has portrayed anyone who appears to stand in its way as an enemy of the people, to borrow the Daily Mail’s pungent phrase. Today, Boris Johnson launched his last week of election campaigning by trailing a speech in which he accuses Corbyn of a “great betrayal” over Brexit.

The Tory party is so dependent on big money it now represents only a tiny elite | Peter Geoghegan Read more

The broad impact of this stab-in-the-back rhetoric has been to toxify political debate, but it has more acute effects too. Since 2016, we have seen the murder of one Labour MP by a far-right conspiracy theorist who proclaimed “death to traitors”, and a neo-Nazi plot to murder another MP. A far-right social media activist has been convicted of harassing a former Conservative MP who opposes Brexit; he also called her a “traitor”. In recent weeks, three Labour canvassers have been attacked in the street: among them, a woman in her 70s in Herefordshire whose assailant ranted about Marxists, and a man of similar age in Rotherham who was taken to hospital with a suspected broken jaw.

Abusive language has flown in many directions as the UK’s politics has become more turbulent, but what’s coming from the right is of a different order altogether – not least because it is being boosted by several mainstream newspapers. What is particularly worrying about the Sun’s publication of a toxic conspiracy theory is that the story was written by its political editor – a veteran journalist with years of experience as a defence correspondent who appears frequently on the BBC. It’s not good enough for the Sun to simply remove it – the paper needs to apologise, and to explain what it got wrong and why.

• Daniel Trilling is the author of Lights in the Distance: Exile and Refuge at the Borders of Europe, and Bloody Nasty People: the Rise of Britain’s Far Right