Reform has made a formal complaint to the police about alleged cheating in the Dorton and Genton by-election. It follows claims of widespread “family voting”, which involves people going into a voting booth together instead of voting separately. This is against the law and raises concerns about people being coerced to vote a certain way by relatives such as spouses.

Reform came second to the Greens in Thursday’s poll but party leader Nigel Farage has written to watchdog the Electoral Commission insisting there are “grave questions about the integrity of the secret ballot and public confidence in the result”. Mr Farage said: “Reform has today reported the many cases of ‘family voting’ to the electoral commission and the police."

The Reform UK leader said: "What was witnessed yesterday is deeply concerning and raises serious questions about the integrity of the democratic process in predominantly Muslim areas.

“If this is what was happening at polling stations just imagine the potential for coercion with postal votes.

“If action isn’t taken now, then we will ensure it is after the next General Election.”

A group called Democracy Volunteers claimed it had witnessed “concerningly high levels” of family voting in Thursday’s election, although Manchester City Council said no issues had been reported.

An Electoral Commission spokesperson said: “Electoral offences are a matter for the police.

“We encourage anyone who believes an offence has occurred to report it to the police.”

Asked if the Prime Minister had concerns about the allegations, a No 10 spokesman said: “The law is the law, and it’s essential that electors can cast their votes in secret without any risk of coercion.

“Obviously, if anyone has witnessed coercion, they should discuss that matter with their local elections team or the police.”

Transport Secretary Heidi Alexander said the reports were “concerning”.

She told Sky News: “People should be able to vote in a free way and not subject to any coercion, and so it would be right that the appropriate authorities, including potentially the Electoral Commission, look into those reports.”

Reform UK leader Mr Farage had earlier said in a post on X: “This election was a victory for sectarian voting and cheating.”

Asked what he meant, his party’s chairman David Bull told Sky News: “The Green Party appealed to the Muslim vote. 33% of that constituency is Muslim. They printed their leaflets in Urdu.”

He added that his party were also worried about family voting and postal votes.

“I’m worried about electoral fraud generally.

“Am I saying it changed the outcome of this election?

“No, but I think what we do need to do is to make sure that these elections, every election, is free, democratic and fair,” he said.

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A Greens spokesperson said: “This is an attempt to undermine the democratic result and is straight out of the Trump playbook.

“We’ve just won a historic by-election by a comfortable margin. We’ve shown the country that Greens can beat Reform, despite their big business donations.”

Green Party leader Zack Polanski rejected any accusations of sectarianism as he was asked whether he had deliberately fuelled anger among Muslim voters with leaflets featuring images of Sir Keir Starmer shaking hands with Indian prime minister Narendra Modi.

While he accepted that Sir Keir needed to speak to world leaders for diplomatic reasons, he said the materials showed his “pally” relationship with them. Conservative Party leader Kemi Badenoch said Labour, the Greens and Reform had all stirred up “grievance politics”.

“If you stir up grievance politics between groups based on religion or race, as Labour have done for decades, as Reform are seeking to do, and as the Greens have done successfully in this by-election, you are pitting neighbours against each other and you start to unravel the culture of tolerance that makes Britain great,” she said.