SYDNEY, Australia — After months of uncertainty, Australia’s High Court declared Friday that five federal lawmakers who have dual citizenship were ineligible to run for their seats, a ruling that threatens the government’s one-seat majority in the House of Representatives.

The decision means that the seat belonging to Deputy Prime Minister Barnaby Joyce of the National Party, a coalition partner, will be subject to an election, which will be Dec. 2.

The ruling came months after what initially appeared to be a minor scandal forced several politicians to resign, eventually growing to present a challenge for the government of Prime Minister Malcolm Turnbull. The issue has raised questions about the vetting of candidates and how Australia’s Constitution is interpreted.

Under Section 44 of the Constitution, citizens of other countries are not eligible to run for the federal Parliament. Constitutional experts say the wording originally referred to people outside the Commonwealth, which included not only Australia but also Canada, New Zealand and a host of other British dominions and territories.