Operation Hooper has involved four years of work by a core team of Cambridgeshire detectives who trawled through more than 11,000 electoral documents and interviewed nearly 1,200 witnesses.

The £1 million investigation began in June 2004 after Peterborough City Council began receiving complaints from people who had arrived at polling stations to vote in the local elections, only to be told their ballots had already been cast by post.

The vast majority of complaints came from the city's Central ward, where electoral records showed the number of postal votes was far higher than in any other area.

In particular, the number of postal votes sent to away addresses – addresses other than voters' homes – was a cause for concern.

A total of 591 away address applications were made in the Central ward – more than 13 times higher than the next largest figure of just 44 in Paston.

Operation Hooper began with the seizure of thousands of electoral documents from the city council, and after the papers were painstakingly analysed by forensics officers and a handwriting expert, six suspects were identified.

Detective Sergeant Shaun Sutton, the deputy senior investigation officer in the case, said: "The initial stages of the investigation took many months, and from the analysis of the documents, we could see there were a number which contained the same handwriting style.

"This then led us to the six people who were to become the subject of the inquiry, and it was only then that the full scope of the investigation became apparent."

Of the nine candidates who stood for election in the Central ward at the June 2004 election, five were brought before the courts in connection with illegal activity, including two of the three successful candidates.

Despite being ousted at the election, Mohammed Choudhary, who became Peterborough's first Asian mayor in 1996, was convicted of four counts of forgery following a four-month trial which concluded in February.

He was sentenced to nine months in jail in April, though he has since been released.

Labour candidate Maqbool Hussein and party aide Tariq Mahmood were jailed alongside Choudhary after being convicted at the same trial.

Mahmood, who was found guilty of 14 counts of forgery, was given a 15-month jail sentence and is still in prison, while Hussein was given a three-month term after being found guilty of four counts of forgery.

Former mayor Akhtar was found guilty of one count of forgery on Friday, while fellow Conservative candidate Razaq was convicted of six charges the previous day.

Both won their seats in the 1994 election.

Failed Tory candidate Mohammed Khaliq was due to be tried alongside Akhtar and Razaq, but pleaded guilty to two counts of forgery on the day the month-long trial was due to start.

All three have been told by Judge Alasdair Daroch that a jail term is likely when they are sentenced later in the year.