Official observers tonight raised the alarm about 'high levels' of illegal 'family voting' in the Gorton and Denton by-election.
Democracy Volunteers, who sent accredited observers across the Greater Manchester seat on Thursday, expressed fears about major breaches of electoral law.
Family voting is where a voter is accompanied by another person into or near polling booths with the intention of influencing their vote.
Tougher legislation, known as the Ballot Secrecy Act, was introduced in 2023 in a bid to clamp down on the practice.
It made it more clearly an offence for someone to accompany a voter to a polling booth or position themselves nearby with the intention of influencing a voter.
In a statement issued as polls closed in Gorton and Denton at 10pm on Thursday night, Democracy Volunteers said they had seen 'the highest levels of family voting at any election in our 10-year history of observing elections in the UK'.
But Manchester City Council insisted 'no such issues have been reported'.
Labour won Gorton and Denton at the 2024 general election with more than half the vote, but polling showed Thursday's by-election to be a three-way contest.
Both Reform UK and the Green Party have high hopes of winning the seat, which is sharply divided in ethnicity, religion and deprivation. A result is expected in the early hours of Friday morning.
Reform leader Nigel Farage said the report of high levels of family voting 'raises serious questions about the integrity of the democratic process in predominantly Muslim areas'.
Official observers have raised the alarm about 'high levels' of illegal 'family voting' in the Gorton and Denton by-election
Matthew Goodwin, the Reform candidate in the by-election, said: 'Given the reports we are reading in UK media about family voting and sectarianism, I am deeply concerned about the extent to which the Gorton and Denton parliamentary by-election is a free, fair, and democratic election.'
Labour chair Anna Turley told BBC Newsnight the reports of family voting were 'extremely worrying and concerning'.
Green Party leader Zack Polanski said: 'I think it's important that there's full transparency about the democratic process.
'And if the recommendation is that there should be an inquiry or further steps then yes I'd support that.'
The four accredited election observers from Democracy Volunteers saw family voting in 15 of the 22 polling stations they attended, the organisation said.
This was 32 cases in total, with nine cases observed in one polling station alone.
It meant that 12 per cent of the voters observed either caused or were affected by family voting.
John Ault, director of Democracy Volunteers said: 'Today we have seen concerningly high levels of family voting in Gorton and Denton.
'Based on our assessment of today's observations, we have seen the highest levels of family voting at any election in our 10-year history of observing elections in the UK.'
'We rarely issue a report on the night of an election, but the data we have collected today on family voting, when compared to other recent by-elections, is extremely high.
'In the other recent Westminster parliamentary by-election in Runcorn and Helsby we saw family voting in 12 per cent of polling stations, affecting 1 per cent of voters.
'In Gorton and Denton, we observed family voting in 68 per cent of polling stations, affecting 12 per cent of those voters observed.'
The organisation said their team attended 22 of the 45 polling stations in the constituency, spending between 30 and 45 minutes in each, and observed a sample of 545 voters casting their vote.
The team also observed cases of voters being turned away, however, in each case this was due to them not being a registered voter for Westminster elections, such as having EU citizenship, and only being allowed to vote in local elections.
Democracy Volunteers also saw a number of voters taking photographs of their ballot papers and one voter being authorised to vote despite them already having been marked as voted earlier in the day.
They informed elections staff at Manchester City Council about their findings, but the local authority said 'no such issues' had been reported.
A spokesperson for the acting returning officer said: 'Polling station staff are trained to look out for any evidence of undue influence on voters. No such issues have been reported today.
'If Democracy Volunteers were so concerned about alleged issues, they could and should have raised them with us during polling hours so that immediate action could be taken.
'We have operated a central by-election hub which has been rapidly responding to reported issues during the day, in liaison with the police - who had a presence at every polling station - where necessary.
'It is extremely disappointing that Democracy Volunteers have waited until after polls have closed to make such claims.'
