Under the Australian constitution, dual nationals are not allowed to be elected into the parliament.
Australia's High Court ruled on Friday that Deputy Prime Minister Barnaby Joyce is ineligible to remain in parliament.
The court’s decision has cost the coalition government its one-seat parliamentary majority.
Joyce was one of seven politicians whose eligibility to sit in parliament was thrown into doubt when it was found they were dual citizens.
According to Australia's constitution, dual citizens are barred from being elected to the national parliament.
The court ordered that a by-election must be held for Joyce's electorate.
Speaking in the rural New South Wales town of Tamworth, Joyce told reporters he respected the court’s verdict.
"I respect the verdict of the court," he said. "It's a pretty simple story, we're off to a by-election."
The Australian dollar fell a quarter of a US cent after the court announced its decision.
TRT World's Caitlin McGee reports.
Prime Minister Malcolm Turnbull now needs to win the support of one of three independent lawmakers to keep his minority government with two sitting weeks of parliament left until recess for the year.
The remaining six politicians, from both the ruling coalition and minority parties, are senators and their seats would be assigned to alternatives from their political party if they are ruled ineligible.
The seven lawmakers accept they were dual nationals at the time of their election last year but the government argues that five of them, including all three Cabinet members, should be cleared because they were unaware that they had contravened the constitutional requirement at the time.
Australian Solicitor-General Stephen Donaghue has urged the seven justices of the High Court not to interpret the constitution literally.