Several police forces across England are probing allegations of electoral fraud.
The investigations include both alleged fraudulent postal voting and false registration of voters' addresses.
Greater Manchester Police said it had 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.
Detectives were also investigating allegations of postal voting fraud in a local election in Peterborough, Cambridgeshire. A spokeswoman said a 39-year-old man had been arrested on suspicion of election fraud and released on police bail without charge until June, pending further investigation.
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. 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.