Officers received reports, understood to relate to issues over paperwork involving at least one Conservative candidate at last week’s local council elections, and have confirmed they are now looking into the matter.
The probe concerns Coun Daniel Dewhurst as election agent, who is also Tory councillor for Lea and chairman of Preston Conservative Association.
A spokesman for Lancashire Police said: “We can confirm that we have received information regarding an alleged electoral irregularity and we are in the process of looking into the matter.
“These enquiries are on-going.
“Lancashire Constabulary works very closely with Returning Officers and electoral administrators to tackle electoral fraud and integrity issues, and ensure elections are delivered in an open and fair manner.
“Any allegations made will be investigated appropriately.”
It is understood the nature of the allegation revolves around signatures on paperwork involving at least one Conservative candidate, who subsequently did not win seats.
A spokesman for Preston Council said: “It’s our role to ensure the professional, fair and impartial conduct of elections and the democratic process.
“Where we receive any information or allegations of electoral irregularity then, as a matter of course, we immediately refer these to the Police to carry out the necessary enquiries.”
Leader of Preston Council, Labour Coun Peter Rankin, said he was shocked by the allegation.
He said: “In the many years I’ve been in local politics in Preston, I’ve never known anything like this.
“The Labour Party is extremely careful about nominations and about asking people to nominate candidates and ensuring people’s signatures match their name on the electoral roll, because we know our electoral services people are very hot on ensuring the nomination papers are absolutely spot on.
“We are always very careful about getting everything right.
“As I understand, the police are dealing with it and no doubt it will come before the council standards committee in time if anything is proven.”
A Conservative spokesman said: “Following an allegation made against Coun Dewhurst, Preston Conservative Association has followed party guidelines in dealing with this matter and are awaiting the outcome of an investigation by the relevant authorities.
“We do not feel that it would be appropriate to comment further until the investigation has been concluded.”
Coun Dewhurst did not respond to a request for a comment.
No arrests have been made.