The far-right group Britain First has boasted of gaining hundreds of new membership applications after Donald Trump shared anti-Muslim videos it had posted online, sparking a transatlantic row with Theresa May.
Britain First leader Paul Golding also said the group's Facebook posts were reaching hundreds of thousands more users.
The Prime Minister has said Mr Trump was "wrong" to retweet videos posted by Britain First's deputy leader Jayda Fransen, which she condemned as a "hateful organisation" dedicated to spreading division and mistrust.
The extraordinary flare-up between the two key allies came after Mr Trump responded directly to her assertion - originally made through her official spokesman - that his re-postings had been wrong.
In a trademark tweet, he wrote: "@Theresa_May, don't focus on me, focus on the destructive Radical Islamic Terrorism that is taking place within the United Kingdom. We are doing just fine!"
Mrs May has since rejected calls to cancel his controversial state visit, despite an outcry from MPs, insisting she remained committed to the "special relationship" between the UK and US.
Pressed on whether the president's state visit should still go ahead, Mrs May said they had "yet to set a date".
Golding, who is facing charges of religiously aggravated harassment alongside Ms Fransen, said the row had boosted Britain First's popularity.
The 35 year-old told the Times: "We have had hundreds of new membership applications and our organic Facebook reach (number of unique users) has increased by hundreds of thousands."
White House press secretary Sarah Huckabee Sanders on Thursday rejected suggestions Mr Trump's comments had "elevated" a far-right group little known outside the UK.
"I think what he's done is elevate the conversation to talk about a real issue and a real threat and that's extreme violence and extreme terrorism, something that we know to be very real and something the president feels strongly about talking about and bringing up and making sure is an issue every single day," she told a White House press briefing.
In pointed remarks apparently aimed at Mr Trump on Thursday, Mrs May said it was essential to deal with terrorism and extremism "from whatever source they come".
Asked whether she regarded the president as a "supporter and enabler" of far-right groups, Mrs May said: "We must all take seriously the threat that far-right groups pose both in terms of the terrorist threat that is posed by those groups and the necessity of dealing with extremist material which is far-right as well.
"I've commented in the past on issues in the United States on this matter. In the United Kingdom we take the far-right very seriously and that's why we ensure we deal with these threats and this extremism wherever it comes and whatever its source."
In a further sign of the seriousness of the row, Britain's ambassador to Washington, Sir Kim Darroch, said he had raised the Government's concerns with the White House.
Brendan Cox, whose MP wife Jo was murdered by a man shouting "Britain first", praised politicians and media figures for criticising Mr Trump, tweeting: "Can't remember the last time everyone from @jeremycorbyn to @theresa_may, @DanielJHannan to @HackneyAbbott, @OwenJones84 to @piersmorgan, has been on the same side.
"Thanks @realDonaldTrump for reminding us that we have #moreincommon and no time for hatred."
Fransen, 31, from Penge, south-east London, is on bail facing trial over four charges of causing religiously aggravated harassment as part of a Kent Police investigation into the distribution of leaflets and the posting of online videos during a trial held at Canterbury Crown Court in May.
She will go on trial at Folkestone Magistrates' Court on January 29, alongside Golding, who faces three similar charges.
Fransen will also appear in court in Northern Ireland in December charged with using threatening and abusive language in connection with a speech she made at an anti-terrorism demonstration in Belfast on August 6.
Meanwhile, Theresa Scrivener, the Bognor woman to whom Mr Trump initially misdirected his tweet to Mrs May, told the Press Association she was now awaiting an apology for the White House. "I'm just glad he was not contacting me to say he was going to war with North Korea," she added.