The Cook Islands Party has been reduced to a minority government and its rival the Democratic Party is considering a constitutional challenge to its right to rule.
This comes after a Court of Appeal ruling voiding the election result in the Aitutaki seat of Vaipae-Tautu.
The party's Moana Ioane, who had been the education minister, had faced a petition and appeal from the Democratic Party's Kete Ioane.
The court has now ordered a by-election.
The decision means the Cook Islands Party has 11 seats in the 24-seat parliament, the Democratic Party 10 and One Cook Islands two.
The Democrats' leader, Wilkie Rasmussen, says they now intend looking into the law books on constitutional issues.
"Or whether the Queen's Representative could say well I am sacking that previous government because it doesn't hold the majority and I will install the government that demonstrates to me 12 members, I suppose, and then that new government will set the process of confirming that in parliament. I think that will be the next step that we are be looking at."