A former DUP mayor of Coleraine has pleaded guilty to six charges of electoral fraud at the 2005 general and local government elections
Dessie Stewart, who originally denied the charges, will be sentenced later.
He admitted four counts of pretending to be someone else in order to cast postal votes, and two of fraudulently stopping free exercise of a proxy vote.
The DUP said a party disciplinary panel would "review and adjudicate" the circumstances Stewart found himself in.
A party spokesman added the DUP takes a "very serious and dim view" of electoral fraud and other such matters.
The charges relate to the last general and local government elections which were held on the same day in May 2005.
A pre-sentence report is being prepared and Stewart will be sentenced on 24 October at Antrim Crown Court.
A councillor since 1989, Stewart was re-elected to council with 773 votes in The Skerries area which includes his home town of Portrush.
Sinn Fein and the SDLP have both called for Stewart to resign from the council.
Sinn Fein councillor Billy Leonard said he could no longer operate as an elected representative.
"This exposes the DUP's hypocrisy of trying to lecture others on democracy and a host of other issues.
"Dessie Stewart has admitted that he pretended to be someone else just to get some more votes.
"He has totally contradicted the whole idea of people entrusting their democratic vote and I think he is now totally compromised."
John Dallat of the SDLP said the case "must come as a huge embarrassment to the DUP".
"In the complaints made to me, I know that one elderly lady was distraught that her vote was used to promote the aims and objectives of a party which she feels has made a huge contribution to the negative politics which she has lived through.
"Councillor Stewart's replacement should go to the candidate who was runner-up in the council elections and most certainly not to the DUP."
Chair of East Londonderry Alliance Association, Paddy McGowan, said Stewart should resign immediately and 2005 election runner-up, Alliance candidate Barney Fitzpatrick, should be co-opted onto the council.
"Any sense of natural justice would not see one discredited DUP councillor replaced by another member from the same party," he said.
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