The Brexit Party has called for an investigation into Labour's narrow victory in the Peterborough by-election.
Labour's Lisa Forbes won the seat on 6 June by 683 votes, ahead of the Brexit Party.
Its chairman Richard Tice said there had been numerous "rumours" about the outcome, including "vote-rigging".
Cambridgeshire Police said no offences had been revealed in its inquiry into bribery and postal votes.
Mr Tice told a press conference earlier there was evidence that Tariq Mahmood had acted as an agent for the Labour Party - a claim Labour dismissed as "nonsense".
Mr Tice said the Brexit Party wanted "answers" and would lodge a petition under the Representation of the People Act 1983 later this week.
"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?" he added.
Mr Mahmood was convicted in 2008 of fabricating votes for the 2004 Peterborough council election.
A Labour spokesperson said he "was not involved in the running of Labour's [Peterborough] campaign in any way".
Brexit Party leader Nigel Farage told the press conference the postal voting system is "wide open to corruption, to intimidation, to bribery, to abuse on a whole number of levels".
He added: "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."
Cambridgeshire Police continues to investigate two allegations - one of a breach of the privacy of the vote and one of the burning of ballots.
But the force has said no offences were found over allegations of bribery relating to postal votes.