Jacqui Lambie has quit the Senate, after telling colleagues late on Monday there is no need to refer her case to the High Court if it is confirmed she has breached section 44 of the constitution by holding dual citizenship.
Senator Lambie, whose father was born in Scotland, has been checking with the British government after concerns were raised about her citizenship status, and had already warned parliamentary colleagues that she would likely be leaving.
On Monday, the Tasmanian independent senator was awaiting urgent advice from British authorities as to whether she was a dual citizen.
Senator Lambie told Fairfax Media she had sought urgent advice last week as to whether she holds dual citizenship, and said she will resign from the Senate immediately if that is confirmed.
"Tasmanians will be the first to know," she said at lunchtime on Monday. She said she hoped to receive the advice within the next 24 hours.
Senator Lambie dismissed the rampant speculation around Parliament House that she was about to resign on Monday afternoon, after Australian Conservatives leader Cory Bernardi told the chamber he knew of another senator under a citizenship cloud.
"I'm not resigning," she said, but clarified she would quit straight away if she does hold dual citizenship.
Senator Lambie last week said she had "no concerns" about her eligibility to sit in Parliament after Tasmania's Examiner newspaper reported her father was born in Scotland, prompting questions about her inherited citizenship.
"I'm happy to put on record that I'm satisfied that my parents are both Australian citizens and I have no concerns about me being a dual citizen because of where they were born or came from - in the case of my father, as an infant," she said at the time.
The ever-broadening citizenship crisis saw three new senators sworn in on Monday, including Fraser Anning, who spectacularly quit One Nation just minutes before he was sworn in.
Meanwhile, Labor and the Coalition reached a deal requiring MPs to declare their parents' and grandparents' ancestry by December 1, in an attempted quasi-audit of MPs' citizenship.
They will also be required to provide details about how they have satisfied themselves they are not dual citizens and what they have done to renounce any foreign ties.
The citizenship crisis has so far claimed two lower house MPs - Barnaby Joyce and John Alexander - forcing December byelections in their seats of New England and Bennelong respectively, and plunging the Turnbull government into precarious minority rule.