Matt Canavan has quit Cabinet amid citizenship doubts surrounding his mother’s Italian ancestry.
Senator Canavan was the minister for resources and Northern Australia.
His resignation will force Prime Minister Malcolm Turnbull to reshuffle his frontbench and may prompt a recount of the Queensland Senate vote.
It comes just weeks after two Greens senators, Larissa Waters and Scott Ludlam, resigned from federal Parliament due to their own dual citizenship.
When Mr Ludlam resigned, claiming he had no idea he was a New Zealand citizen, the Prime Minister described it as a “remarkable oversight”.
The constitution disqualifies potential candidates from election if they hold dual or plural citizenship.
Section 44 of the constitution may have claimed its fifth victim this Parliament, including Rod Culleton and Bob Day, who resigned for different reasons.
Former LNP senator Joana Lindgren is the next candidate on the LNP ticket for Queensland and could be the person to replace Senator Canavan if he is disqualified.
Ms Lindgren has Aboriginal heritage — she is the great niece of the first Aboriginal senator Neville Bonner, who also represented the Liberals in Queensland.
She served a year in the Senate from 2015 but was not re-elected at least year’s double dissolution.