A knock at the door of a semi-detached house on a street in the East ward of the city of Peterborough.

It's the afternoon of April 22, eight weeks before the June 6 by-election in which Labour will squeak to victory ahead of Nigel Farage's nascent Brexit Party.

Alone at home is a 54-year-old Asian woman with severe health problems.

On her doorstep she allegedly finds Dr Shabina Qayyum, Labour candidate for the ward in the local council elections, along with two male party officials.

Comrades in arms: Jeremy Corbyn, with Parliamentary candidate Lisa Forbes, shakes hands with Tariq Mahmood, far left. Some within the Asian community call Mahmood 'Wormy', a nickname that is as uncomplimentary as it sounds

What happened next is in dispute. But the resulting allegations were serious enough for the council to call in the police.

Criminal claim was followed by counter-claim; the only certainty is that politics in Peterborough is a dirty business — and has been for some years now.

Labour's victory by 683 votes, in one of Britain's most keenly-fought by-elections of recent years, has been overshadowed by accusations of electoral fraud and voter harassment — centred on the Asian community which is particularly active in Peterborough politics.

These allegations have hardly been silenced by the presence in Labour Party-related social media photographs of one Tariq Mahmood.

Mahmood, a local businessman who runs a large letting agency, was pictured at the count for the local elections with his hand resting, in apparent benediction, on the head of the victorious Dr Qayyum.

He was also photographed in the company of Labour leader Jeremy Corbyn and his candidate Lisa Forbes as they pressed the flesh on local streets during campaigning for the by-election.

Mahmood with Labour councillor Dr Shabina Qayyum. For those who treasure the sanctity of the democratic process, his continued presence at counts or on the hustings in 2019 is like seeing a wolf in a field of sheep. There is no suggestion that Mahmood was on the doorstep with Dr Qayyum on April 22

They, too, were pictured with Dr Qayyum. (There is no suggestion that Mahmood was on the doorstep with Dr Qayyum on April 22).

Why was this an issue? Because 11 years ago Mahmood was jailed for his role in one of the worst electoral frauds to be perpetrated in the UK this century.

For those who treasure the sanctity of the democratic process, his continued presence at counts or on the hustings in 2019 is like seeing a wolf in a field of sheep.

It is not as if he is obscure or peripheral. He is a 'kingpin' with many local powerful connections. His brother-in-law is the chairman of the Ghousia mosque where Mr Corbyn spoke during a recent visit, much to the ire of local Muslims who complained that such places should be above politics.

Some within the Asian community call Mahmood 'Wormy', a nickname that is as uncomplimentary as it sounds.

What is the local Labour Party thinking? They have claimed that Mahmood had no role in their by-election campaign. Yet this is not quite the impression we found among his local neighbourhoods.

The opportunity that postal votes afford for election fraud has been shown in Peterborough before — by both Labour and Tory activists. And at the heart of the Labour Party plot in 2004 was Tariq 'Wormy' Mahmood. The city's Cathedral Square is pictured above [File photo]

His continued presence in local politics is perhaps a symptom of a wider problem. When we visited the city this week we found a climate of unease, if not fear.

Opposition councillors claimed they had been targeted with vandalism and violence. Some were afraid to speak openly. There were allegations — denied by those said to be responsible — of votes being bought.

Many of the claims involve the practice of giving people a postal vote on demand, an innovation introduced in the Representation of the People Act 2000. No special reason has to be given for applying for a postal vote and it was not until 2007 that a signature and date of birth were required as proof of identification.

The opportunity that postal votes afford for election fraud has been shown in Peterborough before — by both Labour and Tory activists. And at the heart of the Labour Party plot in 2004 was Tariq 'Wormy' Mahmood.

That year's local elections in the city would become notorious for their corruption. Word of what had happened in city centre wards soon reached the authorities.

Voters claimed they had gone to the polls to be told that their vote had already been cast. The city council went to Cambridgeshire Police. In June 2004, Operation Hooper was launched by its major investigations team.

Detectives focused on the Central, Park and Ravensthorpe wards. By the time the prosecutions were completed in autumn 2008, six men had been jailed, including two former mayors.

At the time of the elections, Mahmood was the secretary of a local Labour Party branch.

Another female Asian voter told the Mail about being pressured recently on her doorstep by Labour canvassers. On two occasions, she said, they offered to help her secure postal votes for her household. They also inquired about getting postal votes for her parents, who lived elsewhere. A stock image of the city centre is pictured above [File photo]

He and his co-conspirators — a failed candidate named Maqbool Hussain and Mohammed Choudhary, Peterborough's first Asian mayor — were arrested and charged with forgery and conspiracy to defraud. They denied the allegations.

'They got hold of postal and proxy votes which belonged to voters in the Central ward,' prosecuting QC Anthony Leonard would later tell King's Lynn Crown Court.

'They arranged for postal ballot papers to be sent not to the voter but to addresses with which each defendant was connected.'

Such vote-rigging was fairly simple if one knew the bureaucracy. A would-be fraudster might consult the marked registers that were available after every election. These showed who had actually voted.

The fraudster could note eligible residents who had not used their votes — and so might not in the future — and apply for postal votes in their names. These documents did not need to be sent to the voters' home addresses.

Once received at an address convenient to the fraudster, the postal ballot statements would be marked, before being sent to the unsuspecting Returning Officers. And so democracy was subverted.

All three men were jailed, with Mahmood receiving the longest sentence of 15 months. He was ordered to pay £15,000 in costs.

Later that year, three Tories, including former mayor Raja Akhtar, were jailed for between two and five months over a similar postal ballot fraud.

They had all brought shame on the city. But shame is not something with which Mahmood has been much, if at all, afflicted.

Last year, he made headlines again when he described the Tory Home Secretary Sajid Javid, who has Asian heritage, as a 'coconut' — a derogatory term for a person of colour who has 'betrayed' their race — on social media. (Mahmood insisted the term was not racist and he could just as well have called him 'turnip head'.)

Yet he is still keeping company with such prominent local Labour Party figures as Cllr Dr Qayyum.

Let us return to the house in East ward and the contested account of Dr Qayyum's visit on April 22. A friend of the householder told us the disabled woman was asked her voting intentions. The woman answered 'blue' (Conservative) and said she had a postal vote.

She claimed that Dr Qayyum and her two colleagues then asked to see her postal vote and entered her home. The three allegedly began searching the voter's kitchen to 'help' her find it, the family friend reported.

They were unable to lay hands on the form but the experience left the woman so distressed that she phoned her husband, who was in Pakistan at the time.

Her husband contacted the family friend and asked her to report the matter to the police. The friend, who is also from the Asian community, told us: 'The husband said to me that I should go and help 'Aunty'.

'He told me a lady had been in her house with two men and that they were looking for votes.'

In fact the family friend contacted her local Conservative councillor, Wayne Fitzgerald, who agreed to support the alleged victim in her complaint. He went to the town hall with the householder and her friend to meet a council official.

Cllr Fitzgerald, deputy council leader and chairman of Peterborough Conservative Association, told us: 'I set up a meeting where she could complain to the electoral services manager Rachel Edwards.

'There was a friend who came in with this lady to complain about Dr Shabina Qayyum demanding votes and coming into her home to search for [voting papers] shortly before the local elections.

'Her English was not very good but she relayed a story of being intimidated and pressured to get her postal vote. She felt uncomfortable, threatened and harassed.'

Peterborough City Council duly reported the matter to Cambridgeshire police. On April 25, a police officer attended the complainant's home and interviewed her with the help of a telephone translator.

The family friend said: 'I sat with her during the police interview and there wasn't an official interpreter. The interpreter was speaking to her on the phone in Urdu. They kept saying, 'I can't hear you, I can't hear you.'

'Aunty shakes when she gets nervous and has been hospitalised since this happened. She's still affected by it. I feel so angry.'

The police decided no crime had been committed and have dropped the investigation.

Cllr Qayyum told us the police had not interviewed her about the matter. She denied wrongdoing and threatened to report our reporter to a named local chief inspector, merely for asking about the allegations.

We have since been told by the city council that Cllr Qayyum has made a counter-claim against the Conservatives in relation to the alleged incident on April 22.

A spokesman said the council reported this to police, too. Last night the police said no crime was found to have been committed.

Labour's victory by 683 votes, in one of Britain's most keenly-fought by-elections of recent years, has been overshadowed by accusations of electoral fraud and voter harassment — centred on the Asian community which is particularly active in Peterborough politics. Lisa Forbes is pictured above with Jeremy Corbyn

Another female Asian voter told the Mail about being pressured recently on her doorstep by Labour canvassers. On two occasions, she said, they offered to help her secure postal votes for her household. They also inquired about getting postal votes for her parents, who lived elsewhere.

'They told me, 'If you sign it [the application form] we can take it in for you',' she said, adding that she thought the canvassers could be working for Tariq Mahmood.

The voter, who did not wish to be named for fear of repercussions, went on: 'They think that because you are part of a community they can then pressurise you.'

A 2014 report by the Electoral Commission warned of risks to the 'vulnerable' of being pressured by postal vote fraudsters and recommended that party campaigners should not be involved in assisting others to complete postal votes.

Cllr Fitzgerald also spoke of 'thuggery' among Labour activists and of 'grave concerns' over postal vote harvesting within certain sections of the community.

He said: 'Some Labour supporters placed glass bottles under the wheel of a (Tory) politician's car at the count on election night.

'I bore witness to what happened at the count where this mob from the Labour party were intimidating people. Whatever politics you follow you shouldn't be rude or threatening.'

Of all the ballots cast in the by-election, 29 per cent were postal votes. This is a high figure, but not close to the record for a poll.

A spokesman for the Electoral Commission said: 'We have received emails making allegations in the wake of the Peterborough by-election. Any claims of electoral fraud are a matter for the police and should be reported to them.'

A spokesman for Cambridgeshire police said the force was investigating three allegations of postal vote fraud and one each of bribery and corruption and breach of the privacy of the vote.

Peterborough city council has sought to dispel some of the wilder rumours which have arisen since the by-election. There was no evidence, it said, of postal voters or burka-clad women voting multiple times.

Nor had there been reports of voters being turned away because someone had used their vote illegally. Tariq Mahmood's presence during the campaign was a matter for the Labour Party but he was not at the count in an official capacity.

Last night, the man himself was defiant. Asked about allegations that his associates applied pressure and harassed voters to back Labour, Tariq Mahmood said: 'All false. It is totally incorrect.

'I have not heard anything [about postal-vote pressure] and I do not believe in that. They were canvassing.'

Asked about his role in the local party branch, he said: 'I have no role. It is respect, just respect.'

He said the extent of his involvement was writing Facebook posts and urging his immediate family to vote.

'I have lived and worked in this area for 50 years,' he added. 'I am a Labour man. I speak the truth. I go to the count as I have done for over 30 years and I have a hobby of taking pictures of what is accurate information.

'Nobody has any right to stop me from supporting and going to the count as any member of the public is allowed to do.'

Pressed on where the accusations against him stem from, he blamed the Conservative Party and 'people who don't like me'.

His 2008 conviction, he said, was based on 'not a shred of evidence'. He had simply been 'helping people to vote in the community'.

Mahmood claimed that a Tory involved in the 2004 election frauds has also been campaigning this year. His Conservative critics would have to 'answer to the community'.

And so politics in Peterborough continues much as it did before. A very dirty business.

Additional reporting: Mario Ledwith