The council is co-operating with police
Detectives in Lancashire are investigating an allegation of postal voting fraud.
The police inquiry centres on Blackburn, where voters will cast their ballots in both the general and local elections on Thursday.
A force spokeswoman could only say that police were looking into “irregularity in postal votes”.
Blackburn-with-Darwen Borough Council said it was co-operating with the police investigation.
Lancashire Constabulary has not disclosed whether the allegations concern the local or general election.
“We are currently investigating one case of irregularity in postal votes in the Blackburn area,” the spokeswoman said.
No arrests have been made and inquiries into the circumstances surrounding the complaint are ongoing.
Linda Comstive, Blackburn with Darwen Council’s director of legal and democratic services, said any such complaints were taken “very seriously”.
“We are working with the police on this matter,” she added.