The Brexit Party is demanding an investigation into the hotly contested Peterborough by-election amid allegations of voting impropriety during Labour's narrow victory.

Nigel Farage's party is expected to hand over evidence of alleged wrongdoing in the run up to the contest which took place on June 6 to the police in the coming weeks.

Mr Farage insisted it was not a case of 'sour grapes' after his party was beaten by just 683 votes but a desire to improve the voting system overall which he said is 'wide open to corruption'.

Party chairman Richard Tice said there had been numerous 'rumours' including of 'vote-rigging' in Peterborough.

The Brexit Party wants the existing system of postal voting to be changed before the next election because of fears it can be abused, with voters potentially bribed or intimidated into backing a certain candidate.

Cambridgeshire Police has already said no offences have been revealed in respect to one allegation of bribery and two relating to postal votes during an ongoing investigation into the by-election.

Nigel Farage (pictured today at a press conference held by The Brexit Party in London) said it was not a case of 'sour grapes' as he called for an investigation into the Peterborough by-election

Much of the Brexit Party's concerns about the Peterborough by-election relate to the alleged involvement of Tariq Mahmood in Labour candidate Lisa Forbes' campaign.

Eleven years ago Mahmood was sentenced to 15 months in jail after he was charged with forgery and conspiracy to defraud in relation to local elections in the city.

At the time of the elections Mahmood was the secretary of a local Labour Party branch. He denied the allegations.

He was pictured with Jeremy Corbyn and Ms Forbes last month.

In a presentation to a London press conference on Monday, Mr Tice claimed there was evidence that Mahmood had acted as an agent for the Labour Party in the run up to the by-election.

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 Tice asked: '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: 'There are so many questions, we need answers. The only mechanism available to us to get those answers is to lodge a petition under the Representation of the People Act 1983 and we will be doing that this week.'

Mr Farage said the existing voting system needed to be improved to ensure voters have confidence in it.

Richard Tice, the chairman of The Brexit Party, said 'answers' were needed into allegations of voter fraud as he demanded an overhaul of the current system

Jeremy Corbyn and Lisa Forbes, the newly elected Labour MP for Peterborough, managed to hold onto the seat by just 683 votes after former Labour MP Fiona Onasanya was kicked out

Mr Mahmood (left) was pictured with Jeremy Corbyn and Lisa Forbes, the Labour candidate, the month before polling day for the Peterborough seat

Who is Tariq Mahmood? An ex-Labour branch secretary, Mahmood received a 15-month prison sentence in 2008 for his part in a scheme to fabricate votes at a council election. Six men – three Labour supporters and three Tories – were convicted of electoral fraud after an operation by Cambridgeshire Police relating to a 2004 local election. All were jailed. Dozens of voters in the Central Ward had turned up at polling stations to vote in the city council elections only to be told that they had already submitted postal votes. Last year, Mahmood, who runs a letting agency, was accused of racism for calling Home Secretary Sajid Javid a 'coconut' on Facebook, a racial slur aimed at those deemed to have betrayed their heritage. Mahmood has admitted he canvassed for Labour ahead of the June 6 by-election. But Mahmood, who always denied voter fraud, insisted: 'It is quite wrong to say that I played any significant role in Labour's victory.' Tories claimed that Mahmood had been 'front and centre' of Labour's recent campaign to hold onto the Peterborough seat. But Labour said the claim was 'false and baseless'. Advertisement

'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.'

However, Mr Farage suggested that any changes to the postal voting system should ensure that the option of voting remotely is still available to wheelchair users and overseas service personnel.

Cambridgeshire Police continue to investigate two allegations - one of a breach of the privacy of the vote and one of the burning of ballots - made in relation to the by-election held earlier this month.

But the force has announced that no offences were found over allegations of bribery relating to postal votes.

Postal votes played a major part in the overall result in Peterborough, accounting for almost 10,000 of the 33,998 ballots received.

Some 400 postal votes were rejected because of problems relating to signatures and date of births not matching official records.

The Peterborough by-election was called after disgraced former Labour MP Fiona Onasanya was ousted by her constituents after being jailed in January for lying about a speeding offence.

Labour has previously rejected the suggestion that Mr Mahmood was involved in Ms Forbes' campaign.

They branded the allegations 'false and baseless' and said he was 'not involved in the running of Labour's campaign in any way'.

Commenting on The Brexit Party's latest accusations, a Labour Party spokeswoman said: 'This is nonsense. This person was not an agent for the Labour Party and was not involved in the running of Labour's campaign in any way.

'This is a desperate attempt by the Brexit Party to make excuses for their defeat.

'Labour won the Peterborough by election fairly and squarely. Throughout the campaign we fully complied with the regulations set out by the Electoral Commission and electoral law.'