People Before Profit's Eamonn McCann is seeking an urgent meeting with the Electoral Office following an alleged "breach of rules" by Sinn Fein in polling stations on election day.
Mr McCann last night accused Sinn Fein of malpractice in polling stations, and he claimed that the Electoral Office staff often did little to stop it.
The veteran socialist alleged that Sinn Fein was flouting the rules by communicating information from polling stations about who had voted to party activists outside.
This gave Sinn Fein a massive advantage by enabling the party to mount a "get the vote out" operation, he said.
The Electoral Office could not be contacted for comment about the allegations. However, a Sinn Fein spokesman said: "This is deflection politics by Eamonn McCann.
"The voters of Foyle and West Belfast have once again rejected his party's support for a Tory/Ukip Brexit."
After a recount in Foyle, Sinn Fein took the seat from the SDLP by just 169 votes.
Elisha McCallion secured 18,256 votes to Mark Durkan's 18,087.
The SDLP man had been the MP since 2005.
In his concession speech, Mr Durkan had suggested his party may take action.
Sinn Fein's Stormont leader Michelle O'Neill hailed the result as a victory for her party.
Mr McCann - who was election agent for People Before Profit candidate Shaun Harkin in Foyle - said he felt that decisive action needed to be taken to prevent the alleged malpractices.
In a letter to the Electoral Office, which has been seen by the Belfast Telegraph, he said that it wasn't up to political parties to police their rivals' behaviour in polling stations.
"It is not our job to ensure that election rules are maintained," he wrote.
"That's the job of the Electoral Office.
"We do not doubt the integrity of the Electoral Office staff, but we are disappointed to see that malpractice continues.
"We would welcome a meeting with yourself next week to discuss these concerns and what can be done to make sure that there is no repetition in future elections."
Mr McCann noted that serious concerns about proxy and postal voting arrangements in yesterday's Westminster election had already been raised.
"When this is put together with the concerns evident in polling stations, there is a clear case for a review of the mechanisms in place to give confidence that the ballot is fair.
"There is a strong case for ending the system whereby party agents can sit in the polling booths," he said.
In his letter to the Electoral Office, Mr McCann said: "Our representatives have observed agents of another party sitting in polling booths, and possibly recording information.
"In our view this is in breach of all norms of election procedure."
While the former People Before Profit MLA doesn't name Sinn Fein in the letter, he told the Belfast Telegraph that his complaints concerned that party alone.
The veteran socialist said that PBP representatives were "loudly heckled" by some Sinn Fein workers in polling stations when they raised the issue with officials.
"This has been done within sight and hearing of voters queuing in line for their ballot papers," he said.
He claimed that, in some cases, officials intervened.
Mr McCann said: "The practice of party agents in polling booths noting the electoral numbers of people who have voted and communicating this information to party organisers outside has been a standard feature of recent elections here.
"The purpose being to enable party activists to target and call to the homes of people who haven't yet cast their votes, letting them know that the party knows that they have not voted and offering a lift to the polling station now.
"This compromises the secrecy of the ballot while giving an advantage to the party flouting the regulations."