The High Court has found five federal MPs including Deputy Prime Minister and Federal Agriculture Minister Barnaby Joyce disqualified from sitting in parliament.
Mr Joyce will face a by-election in his seat of New England, putting the Turnbull government's one-seat majority at risk.
Also disqualified were cabinet minister Fiona Nash, former Greens senators Scott Ludlam and Larissa Waters and One Nation senator Malcolm Roberts.
Former cabinet minister Matt Canavan and crossbencher Nick Xenophon were found not to be disqualified.
Section 44 of Australia's constitution bans anyone holding dual citizenship from sitting in parliament, in a section aimed at ensuring MPs do not hold split allegiances.