France's Council of State has struck out election results in French Polynesia's most populated island group.
The constituency includes Tahiti and Moorea and accounts for a total of 37 of the 57 MPs in the territorial assembly.
The ruling effectively triggers a new by-election within the next three months.
France's highest administrative body says there were serious irregularities in the territory's election in May.
The State Council says the decoration of polling stations with party colours in some of the Windward Islands broke rules about neutrality on election day.
France's overseas ministry has reacted to the ruling by asking party presidents and parliamentary caucus leaders for talks.
The ministry says the working session next week is studying legal and technical conditions by which all of French Polynesia's voters could be called back to the polls.
A political stalemate in the territory has halted the workings of government, and has raised fears of a breakdown in public order.
