A conference in Sheffield for the UK Independence Party’s young supporters has been cancelled after a campaign of threats and intimidation by Alt-Left activists, according to its organisers.

“It is with great reluctance that we have been forced to announce that the Young Independence #Horizons conference has been cancelled/postponed indefinitely,” reads an online letter from the YI executive council.

“Unfortunately, the Hard Left Stand Up to Racism organisation planned a protest at the hotel and has sent a large amount of abusive and threatening emails to Hilton Hotel staff. … The Hilton inform us that the quantity of emails that were of an aggressive nature was overwhelming and they decided that owing to an increased security risk posed by the Far Left to delegates, hotel guests and Hilton staff they have been left with very little option than to cancel the event.”

The youngsters had also planned to hire out a boat for a get together after their conference, but this was cancelled as well after Alt-Left activists allegedly “decided that it would be appropriate to send abusive messages” to the small business owner who would have been hosting.

“The lady who runs the boat company informed us that protesters were threatening to cause damage to the boat and although she was personally disgusted with their actions, she simply could not take the serious risk posed by the protesters,” the letter claims.

The group attempted to secure a fallback venue, with the Novotel also cancelling their booking before premises was finally securely in Barnsley.

However, “only three of [the] Leadership candidates and/or UKIP speakers confirmed their attendance”, with two confirming they would not attend and one objecting to the invitation of Martin Sellner, who leads the Defend Europe mission against NGOs and people-smugglers in the Mediterranean, to the conference.

You can't deny Europeans to enter Europe.

You should deny illegals to enter Europe. @MaltaGov#defendeuropehttps://t.co/igVsYI5VLX — Defend Europe (@DefendEuropeID) August 27, 2017

“We defend our decision to invite Mr. Sellner as a person who has experienced first-hand one of the greatest man-made crises in Europe since the Second World War and, arguably, one who attempted to prevent politically motivated NGOs from encouraging human-trafficking and illegal immigration,” the organisers wrote.

“We understand the controversies, however we simply do not believe in no-platforming a person because we don’t agree with every aspect of their views.”

YI noted that some of the UKIP leadership candidates are “not without controversy” themselves, naming, in particular, Anne Marie Waters — a critic of Islam described as “an Irish lesbian feminist” by The Times.

“Naturally, it would be unthinkable for us to exclude her while she is a candidate in the current leadership elections,” they wrote.

Would-be conference attendees who had already made travel arrangements were advised that many planned speakers and organisers will still be in Sheffield and would try to meet with supporters informally.

Nevertheless, YI complained that the state of affairs they find themselves in represents “a massive attack on freedom of speech and reflects the very worrying times we face in this country, where those who have opposing views are able to force events to be cancelled because of their keenness to be violent towards those of an opposing view”.

Fo llow Jack Montgomery on Twitter: @JackBMontgomery