Reform UK leader Richard Tice has claimed the Rochdale by-election was not “free and fair”.
The businessman reeled off a series of allegations including that the party’s candidate Simon Danczuk and his team were subject to death threats, “vile racist abuse” and “daily intimidation and slurs” during the campaign.
He also questioned the rise in postal votes during the contest which has been mired in controversy.
In a post on X, Mr Tice said: “Now the polls have closed, people across the UK need to know the truth about this election campaign – and the implications for our democracy.
“To suggest that a parliamentary election in this country has not been truly free and fair is a very serious allegation indeed.
“Unfortunately however, the behaviour of certain candidates and their supporters in this contest fell very far short of our traditional democratic standards. What we have witnessed and experienced in Rochdale is deeply disturbing.
“In one incident, Reform UK business supporters were threatened with a firebomb attack if they distributed our leaflets.
“Menacing behaviour was a feature of the entire campaign, including outside polling stations on the day of the election itself.
“In this ugliest of contests, we are also concerned by the sudden increase in the size of the postal vote, which has jumped from 14,000 to some 23,000 in this constituency since the last general election.”
Mr Tice said the results of the by-election in the Lancashire town “should act as a stark wake-up call to those in power – and the entire electorate”.
He added: “This is Britain. We are supposed to be a beacon of democracy. This shameful contest has been more characteristic of a failed state.
“Unless something dramatic changes, our fear is that it will be repeated in dozens of constituencies across the UK at the general election.
“By Christmas, we face the prospect of numerous extremist antisemitic lawmakers in the House of Commons.”
Independent candidate William Howarth agreed with Mr Tice that there had been an “element of intimidation” during the campaign.
But left-wing firebrand George Galloway, who won the contest, denied his supporters had engaged in any intimidation.
The former Labour MP, who stood for the Workers Party of Great Britain and focused his campaign heavily on Gaza, also claimed Mr Tice had invited him to be a Reform UK candidate in a recent by-election.
Mr Galloway told Sky News: “I think Mr Tice has rather lost his balance, and Mr Farage too, and I remind Mr Tice that I have on my telephone a text from him inviting me to be the Reform UK candidate in a by-election not that long ago.
“I’d prefer not to publish it, but if he keeps telling lies about me I will have to tell the truth about him.”
Asked whether his supporters had engaged in bad behaviour, he said: “Absolutely none. Ask the police, ask the police if a single one of our supporters has been arrested or spoken to by them.”
Mr Danczuk came sixth behind Labour’s former candidate Azhar Ali who was dumped in an antisemitism row.
