A burgeoning network of anonymous “news” websites that create viral content and spread populist messages about the UK across the internet has caught the attention of the government.

The first of a major three-part investigation by Politico has shown how “a new breed of hyper-partisan news sites” that promote pro-Brexit, anti-Islam, right-wing messages “has got the government worried”.

The threat these news sites pose is deemed to be so great that Downing Street has set up a special unit to monitor their activity.

“But it is struggling for an effective response” says Politico’s Tom McTague, who quotes one “exasperated Number 10 official” as saying: “Who do you ring? You don’t know who these people are”.

Many of these so-called news sites are registered outside the UK. One example is PoliticalUK.co.uk, which was only formally set-up in Arizona nine months ago. It has nevertheless produced some of the most viral news stories in the UK this year, despite having no named editor and only one reporter using a pen name.

Since it started publishing stories at the end of April, the site has amassed more than 3 million interactions on social media, with an average of 5,000 ‘engagements’ for every story it has published, far more than most national newspapers.

The latest was a story that claimed a Ukip-backed protest against Theresa May’s Brexit deal last Sunday had been wildly misreported by the rest of the British mainstream media.

Despite the fact that claims “tens of thousands” had marched against “Brexit betrayal” were immediately discredited, social media monitoring specialist NewsWhip calculated by Monday lunchtime the story had picked up over six times as many Facebook interactions as an equivalent article from the Daily Mail – the most read English-language news sites in the world.

It follows a pattern of insidious and far-ranging disinformation campaigns operating in the UK and US that have slowly come to light in recent years.

Much of this focuses on Russian “trolls” or fake news factories which aim to exploit political, racial and religious tensions in order to create disunity and drive a specific political agenda.

In Britain, openDemocracy has spent two years investigating the dark money and secretive interests behind the Brexit vote and rise of far-right populist politics.

The Guardian has also published an investigation detailing how the far-right activist Tommy Robinson is receiving “financial, political and moral support from a broad array of non-British groups and individuals, including US thinktanks, rightwing Australians and Russian trolls”.

Robinson, whose real name is Stephen Yaxley-Lennon, has become a cause celebre among far-right activists after he was imprisoned for contempt of court. His sentencing earlier this year prompted a vigorous international Twitter campaign, which saw 2.2 million tweets posted using the #freetommy hashtag.

Analysis conducted for the Guardian by the London-based Institute for Strategic Dialogue found that more than 40% of the tweets came from the US, 30% from the UK and other significant volumes from Canada, the Netherlands and nine other countries.

A separate study of about 600 Twitter accounts, believed to be directly tied to the Russian government or closely aligned with its propaganda, found significant numbers had tweeted prolifically in Robinson’s defence.