Image copyright Facebook Image caption Left to right: Brittany Pettibone, Lauren Southern, and Martin Sellner

Three activists with big social media followings from Austria, Canada and the US have been barred from entering the United Kingdom, in a move that some say is part of a crackdown on the far right.

Brittany Pettibone and her boyfriend, Martin Sellner, were refused entry to the UK when they landed at Luton Airport on Friday. They were detained for two days, and then deported. Another activist, Lauren Southern, was refused entry by the Border Force near Calais on Monday. She had planned to meet with the couple and the former leader of the English Defence League, Tommy Robinson.

Sellner, an Austrian and prominent figure in the anti-migration group Generation Identity, was due to make a speech in Speakers' Corner in Hyde Park. He was the leader of a "Defend Europe" campaign last summer, responsible for targeting boats run by NGOs trying to rescue migrants in the Mediterranean.

On his social media accounts, Robinson says he plans to deliver Sellner's speech in Hyde Park on Sunday.

In a statement about the activists, a Home Office spokesperson said: "Border Force has the power to refuse entry to an individual if it is considered that his or her presence in the UK is not conducive to the public good."

Pettibone, an American, tweeted an image of the letter she says was handed to her by an immigration officer. It states that her planned activities posed "a serious threat to the fundamental interests of society and are likely to incite tensions between local communities in the United Kingdom".

The letter also cites Sellner's possession of leaflets which referenced "possible violence at his speech" and calls Robinson "a far right leader whose materials and speeches incite racial hatred." Pettibone was one of the most prominent online voices spreading the "pizzagate" conspiracy theory which falsely claimed that top Democratic officials were keeping child sex slaves underneath a Washington, DC pizza restaurant.

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Southern says she was questioned under the Schedule 7 of the Terrorism Act 2000, on her political views and her opinion on right-wing terrorism. She tells BBC Trending that she was refused entry on the grounds of her involvement "in the distribution of racist material in Luton".

In February, the Canadian activist displayed flyers saying "Allah is a gay god" outside a restaurant in the town centre. Southern, who has nearly half a million subscribers on YouTube and regularly posts politically charged stunts, says this was part of a "social experiment" video.

Image copyright Twitter Image caption Brittany Pettibone (left), Lauren Southern (middle) and British activist Caolan Robertson (right), at a stall in Luton town centre

Like Pettibone and Sellner, Southern was informed that her actions "present a threat to the fundamental interests of society" by Border Force.

She tells BBC Trending she believes the government has a "political grudge" against Pettibone, Sellner and herself.

"I would hope that [the UK] would be pro-freedom of speech, and support people's right to question Islam, to even have cheeky posters, make jokes and social experiments, to give a speech at Speakers Corner," says Southern.

'Softer' targets?

Nick Lowles, chief executive of the anti-racism campaign group Hope Not Hate, says there has been a shift in who the UK government considers to be a threat. He says that "during the last two to three years the government has taken a very strong position against hard-line neo-Nazis, extreme Holocaust deniers, banning many who've attempted to enter the UK."

"What's new about the banning of Generation Identity activists such as Martin Sellner, Brittany Pettibone, and their increasingly alt-right friend Lauren Southern, is that the government has signalled that it's going after 'softer' targets on the hard right," says Lowles.

"These are people who have huge reach on social media, they are peddlers of online hate, and as our recent State of Hate report highlighted, the online reach of right-wing hate preachers can have disastrous consequences."

"The irony is that the far right have long called for the British government to take firm control of our borders. Now they are doing just that."

The activists were prevented from entering the UK at a time of rising concern about far-right violence. Last month, Mark Rowley, the former head of counter-terrorism policing in the UK, warned of the growing threat of far-right terrorism.

"Islamist and right-wing extremism is reaching into our communities through sophisticated propaganda and subversive strategies creating and exploiting vulnerabilities that can ultimately lead to acts of violence and terrorism," he said.

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