Police have confirmed they are investigating allegations of vote theft in Foyle, days after Sinn Féin dramatically snatched the seat from the SDLP.
Colum Eastwood's party is also set to raise concerns about a second constituency.
Mark Durkan lost his Foyle seat to former city lord mayor Elisha McCallion by 175 votes, and SDLP veteran Margaret Ritchie was defeated in South Down by Sinn Féin's Chris Hazzard by 2,446 votes.
SDLP MLA Mark H Durkan is due to meet the Electoral Office today in a bid to determine how widespread the potential electoral fraud was during last week's British general election.
The Police Service of Northern Ireland said officers are investigating "a small number of reports of electoral fraud, following referral from the Electoral Office of Northern Ireland".
Chief Superintendent Karen Baxter said: "We work closely with the Electoral Office and where information becomes available in relation to criminal activity, we take action."
Police have asked anyone with concerns about electoral fraud to contact the Electoral Office of Northern Ireland.
Mark H Durkan claimed that on polling day and in the days following he and a number of others were contacted by people who said that when they went to vote they were told they had already voted.
He said: "It's difficult to gauge how widespread it may be. Part of my meeting with the Electoral Office is to try and establish how many people this did happen to.
"It is vitally important to all parties that we have an electoral system that people can have faith and confidence in.
"We need to know what steps are going to be taken to eradicate these type of incidents in the future."
In South Down, SDLP MLA Colin McGrath said he had been made aware of "at least a dozen" similar incidents.
"We can't say at this stage if votes had been stolen or anything like that but it is very strange," he said.
"We need to find out how many times this happened and I will be meeting with the Electoral Office to try and determine exactly what happened."
Nobody from the Electoral Office was available to comment.