Councils received a large number of late registrations.
More than 30 allegations of postal vote irregularities have been reported to police forces in England.
In London, 28 allegations have been received by the Metropolitan Police, relating to 12 boroughs.
The force is investigating five - four of which are in Tower Hamlets, while 23 allegations are still being assessed.
West Yorkshire Police are investigating three allegations, while Derbyshire Police say a number of allegations have been received.
Police would not confirm whether the irregularities related to the general election or local elections, which are also taking place across England on 6 May.
The Met said the allegations of electoral irregularities relate to Tower Hamlets, Lambeth, Barking and Dagenham, Westminster, Enfield, Hounslow, Haringey, Ealing, Brent, Bexleyheath, Camden, Redbridge.
A 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."
Tower Hamlets Council said it had passed 10 allegations onto the police for investigation. It said it takes measures beyond most local authorities to combat voter fraud.
The council said it had received 3,123 registrations in the days before the 20 April deadline, which it did not have time to check before the register closed.
It has searched properties where more than eight people were registered and removed 141 people from the electoral register.
The Electoral Commission said the allegations proved new checks to prevent voting fraud, which were introduced in 2006, were working.
Registration officers now have 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 also 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: "The 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."