The seat of Nukufetau has been vacant since sitting MP Lotoala Metia died in December.
The vacant seat means the government currently has seven sitting members in parliament, the same number as the opposition.
The government of Willy Telavi says lawyers are still working on the details of a by-election, which is the cause of the four month delay.
But opposition spokesman, Taukelina Finikaso, has told Radio Australia's Pacific Beat they're taking the matter to the High Court.
"We have indeed raised this with the government more than once," he said.
"Given the island concerned, the island community of Nukufetau, they've also written to the prime minister, being the minister for elections, asking him when will the election take place.
"But he keeps on delaying it and we think that this is deliberately done because he fears that he might lose his parliamentary majority."
Mr Finikaso is also calling on the cabinet to convene the parliament to test the numbers..
He says if the Education Minister, Falesa Pitoi, fails to appear, that would mean the opposition would have seven seats to the government's six.
"He's fallen ill while he was on tour to Cuba and now he's in New Zealand," he said.
"It's going on to four months since he left Tuvalu, and we really don't think he's going to be able to come back and form the government."