If Donald Trump’s presidency has a house publication, it will not be the 165-year-old New York Times, which Trump attacked again on Twitter on Sunday. It will surely be Breitbart, a rightwing opinion and news website formed in 2007 that one former editor has described as “Trump Pravda”.

America’s president-elect has now appointed the site’s executive chairman, Steve Bannon, a former Goldman Sachs banker known for “screaming matches” with staff, as his White House strategy chief.

The fast-growing opinion and news website has become a rallying point for Trump’s nationalist, sometimes racist and often angry “alt-right” support base. It campaigned hard for Trump throughout the primaries, waging war against the candidacy of the Florida senator Marco Rubio, before becoming Trump’s voicebox during the general election.

Donald J. Trump (@realDonaldTrump) The @nytimes sent a letter to their subscribers apologizing for their BAD coverage of me. I wonder if it will change - doubt it?

The Washington website already has outposts in Jerusalem, London, Los Angeles and Texas, and expansion is planned into Germany and France, where it is expected to support Marine Le Pen’s Front National.

“So much of the media mocked us, laughed at us, called us all sorts of names,” said the site’s editor-in-chief, Alexander Marlow on Sunday, following Bannon’s appointment. “And then for us to be seen as integral to the election of a president, despite all of that hatred, is something that we certainly enjoy and savour.”

During the election campaign, Hillary Clinton warned her supporters of the alt-right’s “emerging racist ideology” and “the de facto merger between Breitbart and the Trump campaign … [which] represents a landmark achievement for this group, a fringe element that has effectively taken over the Republican party”.

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And indeed, white supremacist leaders in the US were delighted on Monday that Bannon had secured a White House appointment. According to the Southern Poverty Law Centre’s Hatewatch report, Ken Reed, the national director of the neo-Nazi group the Aryan Renaissance Society, responded to Bannon’s hire with a Facebook post: “Can you say WINNING boys and girls???” He followed with “#WhiteLivesMatter”.

With Trump continuing to use his Twitter account to vent on everything from protesters to the reporting of his policy on nuclear weapons, Breitbart is now poised to become a key outlet for White House communications.

“It will be as close as we are ever going to have – hopefully – to a state-run media enterprise,” Kurt Bardella, a former Breitbart spokesman and now a critic of the website, told the New York Times at the weekend.

Breitbart’s webpages foam with opinion and news divided into themes that appeal directly to the concerns and fears of its rightwing readers. They attack “big government”, “big Hollywood” and “big journalism”. One story from August about plans by Bill Kristol, a Republican considering blocking Trump’s path to the White House, was published under the headline: “Bill Kristol: Republican spoiler, renegade Jew”.

On Monday there was a video of a white man being beaten in the road by a black man while onlookers jeered: “Don’t vote Trump”, plus a report about university students in Washington DC burning a stars and stripes flag.

Ukip leader Nigel Farage has become a regular Breitbart columnist and his former aide Raheem Kassam also worked for the site. Both met Trump at his 5th Avenue penthouse on Saturday night.

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Bannon has said Breitbart trades in “original reporting untainted by establishment spin”, but the tone is angrier than that. It regularly features “shock jock” articles including “Would you rather your child had feminism or cancer?” and “Birth control makes women unattractive and crazy”. Its “wear the #war” merchandise section sells T-shirts emblazoned with the slogan “under attack all the time” and featuring crossed guns.

It is an ethos carried forward from its founder Andrew Breitbart, who died suddenly in 2012, aged 43. Shortly before he died, he recorded a video message that pledged “war” against the liberal establishment. But according to Angelo Carusone, executive vice-president at Media Matters, a non-profit research centre which analyses rightwing media, Breitbart “would have hated the current iteration of his own website” as “he was no racist”.

Carusone first became aware of this “hip new rightwing news outfit advancing misinformation” six years ago. “I consider Breitbart to be a disreputable website,” he said. “It’s a propaganda tool advancing the ethno-nationalist movement.”

Breitbart’s US audience has more than doubled from 7.4 million users in September 2014 to 15.8 million this September. That is still much smaller than the Washington Post and the New York Times, which recorded 83 million and 88 million respectively, according to ComScore, although figures are expected to be higher for November.

“Breitbart has been hugely significant in the US,” said Matthew Elliott, the former chief executive of the Vote Leave campaign in the UK. “Virtually every newspaper endorsed Clinton and, outside of Fox News, all of the broadcasters laughed Trump out of court. In that climate, there was a market for a website that put over a more Trumpish view of politics.

“It’s the voice of the angry right. No other media outlet has reflected this. In the UK we have a more plural press and you can make a choice within the mainstream media.”

Bannon’s leadership of the site has been strongly criticised by Ben Shapiro, the site’s former editor-at-large, who left in March. In an article he republished on Monday, he claimed: “Breitbart openly embraced the white supremacist alt-right … He [Bannon] used his role as Breitbart CEO to turn the outlet into Trump Pravda, creating a stepping stone to close connection with Trump.

“Breitbart publicly burned bridges with everyone to maintain its Trump loyalty. That was Bannon, a scorched-earth personal opportunist.”

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Bannon has been accused of being personally antisemitic in court documents from 2007. His ex-wife said he did not want their daughters attending the Archer school for girls – a private school in Los Angeles – because he did not want the girls going to school with Jews. According the court filings, she stated: “He said he doesn’t like Jews and that he doesn’t like the way they raise their kids to be ‘whiny brats’.”

A spokeswoman told US media that Bannon denied the allegation. “At the time, Mr Bannon never said anything like that and proudly sent the girls to Archer for their middle-school and high-school education.”

But in the hours since his appointment, it was his stewardship of Breitbart that was in the public spotlight. On Monday morning, Trump’s newly appointed chief of staff, Reince Priebus, was asked on MSNBC about “concerns about Steve Bannon because of the Breitbart connections”. “He was a force for good on the campaign at every level that I saw, all the time,” he replied. “I’ve only seen a generous, hospitable, wise person to work with.”

Newt Gingrich, tipped as a possible secretary of state, defended Bannon on Sunday on CBS. Asked about claims that Trump’s victory in the primaries “gave unprecedented visibility to the alt-right, a small but vocal fringe of white supremacists and antisemites and self-proclaimed fascists”, Gingrich said it was “garbage”. “You get this all these smears of Steve Bannon,” he said. “Steve Bannon was a naval officer. He was a managing partner of Goldman Sachs. He was a Hollywood movie producer.”

To Milo Yiannopoulos, Breitbart’s technology editor, who was permanently banned by Twitter for his role in the online abuse of Ghostbusters actor Leslie Jones, Breitbart is engaged in “a culture war” with liberals. “With the cowardly suspension of my account, Twitter has confirmed itself as a safe space for Muslim terrorists and Black Lives Matter extremists, but a no-go zone for conservatives,” he said following his suspension in July. “Like all acts of the totalitarian regressive left, this will blow up in their faces, netting me more adoring fans.”

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When asked how he, Breitbart and other sites mobilised the alt-right community, Yiannopoulos said: “I don’t pander to anyone. I just gave the alt-right a fair hearing. That was considered heretical by the virtue-signalling leftist media, so now they call me a white nationalist and antisemite, despite the fact that I’m a gay Jew with a black boyfriend.

“To the American media, anyone who is not a far-left social justice activist is a racist and sexist. This has had a predictable consequence: no one cares about their name-calling and hysteria any more.

“Trump’s voters, and I would wager in fact most of America, are repulsed by the Lena Dunham, Black Lives Matter, third-wave feminist, communist, ‘kill all white men’ politics of the progressive left. Which is in large part why this election went the way it did. Some of us saw it coming a while ago. Most didn’t.”

The UK anti-racism campaign group Hope Not Hate has been watching Breitbart closely. “Under Bannon, Breitbart has provided a key online resource for the loose collection of ultra-conservatives, neo-Nazis and internet trolls calling themselves the alt-right,” said David Lawrence, a Hope Not Hate researcher. “Following the racially motivated massacre of nine [African American] worshippers in a South Carolina church last year, Breitbart published a piece entitled ‘Hoist it high and proud: the Confederate flag proclaims a glorious heritage’.”

On Breitbart’s UK site, its London editor James Delingpole chimed with Trump’s climate change denials with the headline: “Climate change: the hoax that costs us $4bn a day.” Delingpole claims that “climate change is the biggest scam in the history of the world – a $1.5tn-a-year conspiracy against the taxpayer”.

Delingpole has been described as a “figure of fun” by Bob Ward, policy director at the Grantham Research Institute on Climate Change and the Environment and at the London School of Economics. “He has no scientific credibility, he uses inflammatory language, and resorts to extreme statements.” Although he attacks climate scientists, Delingpole admits to having little scientific knowledge.

In a BBC2 programme about climate change, he told Nobel prize winning scientist Sir Paul Nurse: “It is not my job to sit down and read peer-reviewed papers, because I simply haven’t got the time … I am an interpreter of interpretations.”