Australia's deputy prime minister is ineligible to sit in parliament as he held dual citizenship when elected, the nation's highest court ruled Friday, in a move that threatens the government's slim majority.

Barnaby Joyce was among seven politicians embroiled in a crisis after falling afoul of a previously obscure constitutional rule that bars dual citizens from sitting in parliament.

The High Court decision means the government loses its one-seat majority in the lower House of Representatives until after a by-election for Joyce's seat.