The SDLP has written to Secretary of State Karen Bradley to ask why a by-election has not been called in West Tyrone following Barry McElduff's resignation almost two months ago.
However, sources have told the Belfast Telegraph that Mrs Bradley is planning for the poll to be held on May 3, the same date as England's council elections.
Daniel McCrossan, an SDLP MLA for the area, said the "prolonged vacancy" in the constituency was unacceptable.
The Northern Ireland Office did not respond to requests for comment.
Mr McElduff resigned amid controversy after he tweeted a photograph of himself with a Kingsmill loaf on his head on January 5, the anniversary of the Kingsmill massacre.
Sinn Fein has selected 26-year-old Carrickmore solicitor Orfhlaith Begley as its candidate.
With a 10,000-plus majority in last year's Westminster election, the party is a clear favourite to hold the seat.
So far there have been no moves in Parliament for the issuing of a writ for a by-election.
A poll is usually held between four and five weeks after the writ is issued.
Although the technical procedure to call a by-election is complicated by the fact that Sinn Fein doesn't take its seats in the House of Commons, a by-election was held in Mid Ulster in March 2013, two months after Martin McGuinness resigned as its MP.
When former Prime Minister David Cameron stepped down as Witney MP in September 2016, a by-election was held the following month. A similar timetable occurred when Labour's Sadiq Khan stepped down as Tooting MP after his election as London Mayor.
In previous instances of a Sinn Fein MP resigning, the writ for the by-election has been moved by the Leader of the House.
Mr McCrossan said he was concerned by the delay.
"I've written to the Secretary of State asking her to explain why, two months on from Barry McElduff's resignation, nothing has happened," he said. "As a result of Sinn Fein's abstentionist policy, people in this constituency haven't been represented at Westminster for 10 years.
"But the collapse of the Assembly means an even bigger vacuum has been created in terms of representation, and it is wearing for ordinary people.
"Many are asking when this by-election will actually be called." Mr McCrossan said the SDLP had initially assumed the delay was due to the Stormont talks.
"We believed the thinking was that an election during that period could jeopardise the negotiations. With the talks collapse last month that justification is long gone," he said.
"As a border constituency, West Tyrone faces huge challenges with Brexit and we need our voice heard at Westminster now more than ever."
The overwhelming majority of by-elections are held within three months of a vacancy, although there have been exceptional cases of a six-month gap.
It has been mooted that a victims' campaigner could run against Sinn Fein if the other parties agreed to pull out of the race. But the DUP and the SDLP are unlikely to do so.