The number of investigations has risen from two earlier in the week and includes one message shared on social media allegedly showing someone bragging that he and two others had ‘burned more than 1,000 votes for the Brexit Party’.

Last Thursday’s by-election saw Labour’s Lisa Forbes win the seat by 683 votes ahead of the Brexit Party’s Mike Greene. However, the Conservative deputy leader of Peterborough City Council, Cllr Wayne Fitzgerald, has alleged there was “harvesting” of postal votes, while the Tories have highlighted the involvement in the campaign of Labour activist Tariq Mahmood who was jailed in 2008 for postal vote interference.

Mr Mahmood was pictured with Ms Forbes and Labour leader Jeremy Corbyn during the campaign and was also present at the election count - just as he has been at previous elections.

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A polling station at City College Peterborough at the recent by-election

However, Labour has categorically denied that Mr Mahmood had any involvement in the campaign, with a spokesman saying: “Peterborough council, who organise the count, always announce the results in a public area of the venue. There were hundreds of people there. The Labour Party had 19 official guests who were allowed in to the restricted areas. He was not one of them.

“Members of the public can of course support Labour, but this individual did not play any role in Labour’s campaign.”

The authorities have been keen to stress that so far there has been no evidence of electoral fraud committed, with Peterborough City Council issuing a lengthy statement on its website on Monday after being inundated with queries from members of the public.

Tariq Mahmood

The council, though, which has a dedicated electoral fraud hotline, has now referred five allegations of electoral fraud to police to investigate.

A Cambridgeshire police spokeswoman said: “We are looking into five allegations relating to the by-election and whether any offences have been committed.

“These break down into bribery and corruption x1, postal votes x3 and breach of the privacy of votes x1.”

The investigations are ongoing.

The council last year signed up to take part in a pilot scheme to prevent voter fraud, with certain voters needing to show identification.

However, with three high-profile elections taking place in a five week period the council pulled out of the scheme for this year.

Despite this, the authority insisted that the date of birth and signature which are required on postal voting forms are cross-checked with the same information given when a postal vote application is submitted.

Moreover, a spokeswoman said the matching process takes place on a large TV screen at the Town Hall, with approved party agents able to watch and challenge any postal votes.

Labour has also branded the allegations of electoral fraud as “false and baseless”.