The leader urged an end to the postal votes system in its current form, as party chairman Richard Tice said there had been numerous “rumours” including of “vote-rigging”.

Cambridgeshire police, however, has said no offences have been revealed in respect to one allegation of bribery and two relating to postal votes during its ongoing investigation.

Two more allegations - one of a breach of the privacy of the vote and one of the burning of ballots - are still being investigated.

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Nigel Farage at a Brexit Party rally in Peterborough

Labour’s Lisa Forbes won the June 6 poll by 683 votes, narrowly pushing out the Brexit Party’s Mike Greene into second place.

Tariq Mahmood

In a presentation at a London press conference on Monday, Mr Tice claimed there is evidence that convicted electoral fraudster Tariq Mahmood had acted as an agent for the Labour Party.

Mr Tice called for “answers” and said his party would lodge a petition under the Representation of the People Act 1983 later this week.

Mr Mahmood, who was jailed in 2008 for postal vote interference, has categorically denied the allegations that he campaigned during the by-election, calling them “smears”.

The Labour Party has also denied Mr Mahmood was part of their campaign, while they have branded allegations of electoral fraud as “false and baseless”.

The election saw 9,898 postal votes returned at a rate of 69.6 per cent, with 400 rejected.

Peterborough City Council has said that Mr Mahmood was not a polling or counting agent for the Labour Party, but Mr Tice said: “How much did Lisa Forbes, the elected MP, know that she had a convicted electoral fraudster in her team who would count as an agent?”

The Brexit Party said it would lodge the petition at the High Court on Thursday to ensure a “full investigation” of any allegations.

But the Electoral Commission said the request would have to be a challenge of the election result, while any allegations of criminality are for police to investigate.

Mr Farage said it is time to find out what has happened to democracy.

“Postal votes is a very major part of that,” he said.

“It’s about a system that is wide open to corruption, to intimidation, to bribery, to abuse on a whole number of levels.”

He continued: “If you say it’s sour grapes, you can say it, but actually it is time for change and my ambition would be that by the next general election to get rid of the current postal vote system.”

Wheelchair users and overseas service personnel are among those Mr Farage suggested should keep the postal vote.

Mr Mahmood was photographed inside the room where counting of votes took place, but he insisted that was only when counting had finished and members of the public were allowed in.