Files from 12 police forces relating to allegations of electoral fraud have been handed to the Crown Prosecution Service.
Prosecutors are investigating claims that more than 20 Conservative MPs broke stringent spending limits during the 2015 general election.
If charged, they could be challenged by their political opponents to a rerun.
The expenses challenge centres around the Conservatives wrongly recording spending on a “battle bus” to campaign in certain key seats.
Police forces in Avon, Somerset, Cumbria, Derbyshire, Devon and Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, Metropolitan, Northamptonshire, Nottinghamshire and West Yorkshire sent files. Files from Staffordshire are also reported to have been sent to the Crown Prosecution Service.