A surge in Ukip membership is shifting the party decisively towards the far right, as long-standing moderates are replaced by entrants attracted by an anti-Islam agenda based on street protest, a Guardian investigation can reveal.

Membership has risen by about 50% over the 12 months from a low point a year ago, rapidly reshaping the party in the image of its leader, Gerard Batten, who describes Islam as “a death cult” and has appointed the anti-Muslim activist Tommy Robinson as an adviser.

Conversations with more than two dozen current and former Ukip figures, many of them senior, have uncovered an exodus of more moderate senior members and local organisers, hollowing out Ukip’s ability to fight future elections.

Play Video 8:51 How Ukip normalised far-right politics – video explainer

Sources say many of the 8,000 or so newcomers who have joined in recent months appear to be younger and more radical, attracted both by Robinson and the party’s links to controversial YouTube agitators.

Members of Ukip’s youth wing have posted antisemitic and other extremist messages online, and there has been a rise in the popularity of news websites pushing the party’s message.

Analysts warn that with Ukip’s poll numbers rising amid the continued deadlock over Brexit there is a danger the party could soon be reinvented as a street movement, becoming the first significant far-right force in UK politics since the demise of the British National party (BNP).

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According to party insiders, Batten appointed Robinson, whose real name is Stephen Yaxley-Lennon, without consulting colleagues. They say he is increasingly adopting what one called a “bunker mentality” in which he relies heavily on Robinson, the founder of the English Defence League, and other confidants from the far right.

The Guardian has seen antisemitic messages posted on an unofficial Ukip youth web group, including one saying that life under Hitler was “better than anywhere else on earth”, and another describing Jews as “hook-nosed masters” who control the media. The member involved has since been expelled.

And while Ukip has publicly sought to reject links to the yellow-vest protesters who have harassed politicians and journalists outside parliament and have links to far-right and anti-Islam views, their main Facebook page is run by Martin Costello, the chair of Ukip’s Wiltshire branch and a former parliamentary candidate.

Costello told the Guardian he was helping to coordinate the movement, describing himself as a “modern-day Wat Tyler”.

Since Batten took over as Ukip leader last year, he has proposed new policies including extra checks for immigrants from Islamic countries and Muslim-only prisons, while his families spokesman has said Muslims gangs are responsible for “a holocaust of our children”.

Such changes, together with the appointment of Robinson, prompted a series of MEPs to quit the party, among them the former leader Nigel Farage, who has since launched a new Brexit-based party.

Of the 24 Ukip MEPs elected when the party topped the polls in the 2014 European elections, only seven remain.

Facebook Twitter Pinterest Gerard Batten, the Ukip leader, speaks at the party’s annual conference n Birmingham last September. Photograph: Aaron Chown/PA

Insiders say this change has been reflected at local level, with many party organisers leaving. A source at one of Ukip’s former strongholds, Thanet in Kent, called the party operation there “a shadow of what it used to be”.

But senior members say this has been balanced by an influx of predominantly younger members, many attracted by Ukip’s tie-up with YouTube personalities such as Paul Joseph Watson, from the far-right US conspiracy theory website Infowars.

The party’s membership stands at more than 27,000, well below the figures seen under Farage but 50% higher than in February 2018. Membership jumped 15% in one month alone last summer, and Ukip’s poll ratings have risen to about 7%.

Batten is credited with turning around Ukip after the disastrous tenure of Henry Bolton. Katie Fanning, a member of Ukip’s national executive committee (NEC), calls his achievements “fantastic”.

However, Fanning said Batten had appointed Robinson as an adviser without informing the NEC, describing this as “an unwise choice”.

With so many opponents having left, Batten’s position looks secure, even though he faces a coming leadership contest, having taken over on an initial one-year interim basis.

It remains to be seen how much he can use disquiet over Brexit as well as the reach of Robinson, Watson and others to build Ukip into a stronger political force, whether electorally or otherwise.

Robert Ford, professor of politics at the University of Manchester, said some of Ukip’s recent rise in the polls could be down to voters not realising how much the party had changed.

As an openly far-right organisation, Ford said, Ukip could be “too radioactive” to pick up disaffected Tory voters, who could instead be tempted by Farage. “If Ukip end up as BNP 2.0,” he said, “there’s a very low popularity ceiling on that.”

However, Ford added, the party could thrive instead as a street-focused protest group: “The next couple of years are going to be very, very unpredictable in British politics. Brexit currently occupies all the space in media and politics, and no one quite knows what’s going to rush in when that space is finally vacated.”

David Lawrence, a researcher with the group Hope Not Hate, said Ukip was “now explicitly a far-right party”.

It was possible, he said, that in this form it could eclipse the influence of the BNP: “We are seeing a shift in the political landscape with Brexit, and I think it is significant that Ukip are now very extreme, and polling these kinds of numbers. It certainly speaks to very worrying trends in UK politics.”

A spokesman for the party said Batten had “saved the party from certain financial collapse”.

He said: “Ukip has employed new campaign staff for the upcoming local elections, and candidate recruitment and selection has been in overdrive since Mr Batten put the party on electoral red alert in December. Ukip does not stand for any ‘far-right’ policies, and our manifesto is available for all to read on Ukip.org.”

The spokesman added: “The Guardian is a far-left propaganda organ that is only interested in publishing false information intended to damage Ukip’s electoral prospects.”