Several police forces across England are probing allegations of electoral fraud, it emerged.
The investigations include both alleged fraudulent postal voting and false registration of voters' addresses.
Greater Manchester Police said it received reports of electoral fraud in the Rochdale, North Manchester, Oldham and Bolton areas, relating to breaches of the electoral Code of Practice.
The force said officers were working with the Electoral Commission and local election officers to establish if any crimes had been committed.
West Yorkshire Police are also investigating three allegations of fraudulent postal voting in the Bradford area. No arrests have yet been made, a force spokeswoman said.
Two men aged 60 and 39 from Halifax were held on suspicion of electoral fraud in the Calderdale area last month. They were later bailed pending further inquiries.
In London, the Metropolitan Police said it had received a number of allegations concerning false registration of voters' addresses in Tower Hamlets.
Its four ongoing investigations in the borough were being combined into one investigation, alongside a second investigation into election irregularities in Ealing.
A total of 23 allegations of election irregularities across the capital were currently still being assessed.
The allegations relate to 12 boroughs including: Tower Hamlets, Lambeth, Barking and Dagenham, Westminster, Enfield, Hounslow, Haringey, Bexleyheath, Camden, Redbridge, Hillingdon and Bromley.
No further action was currently being taken based on information received on an allegation in Brent.
A Scotland Yard spokesman said: "All complaints and allegations received will be assessed, and where appropriate, will be thoroughly investigated in close liaison with the CPS and other relevant agencies.
"We would encourage anyone with information regarding suspected electoral fraud or irregularities to report concerns to their local police or local authority."
The Electoral Commission said extra security checks to prevent voter fraud were introduced in 2006 and were now in place for the first time for a general election.
Registration officers have new powers to check people registering to vote are who they claim and, where appropriate, remove them from the electoral register.
While anyone registering to vote by post has to provide their date of birth and signature so this can be checked when their vote is cast.
Peter Wardle, chief executive of the Electoral Commission, said: "Recent reports show these checks in action, with Registration Officers and the police aiming to catch potentially fraudulent applications before polling day.
"The number of reports of allegations should also be seen in the context of the total number of votes - at the 2005 general election over 27 million votes were cast."
Detectives were also investigating allegations of postal voting fraud in a local election in Peterborough, Cambridgeshire.
A spokeswoman said a 39-year-old man was arrested on suspicion of election fraud and released on police bail without charge until June, pending further investigation.
Peterborough City Council said officials had been "working closely" with police.