Deputy Nationals leader Fiona Nash has received advice from the UK Home Office that she may be a British citizen by descent through her Scottish-born father.
She joins the queue at the High Court behind Deputy Prime Minister Barnaby Joyce, who earlier this week renounced his New Zealand citizenship, and Matt Canavan, who stood aside as resources minister in July after learning he has Italian citizenship.
Senator Nash said based on the advice of the solicitor-general, Prime Minister Malcolm Turnbull had advised, that like Mr Joyce, there is no need for her to step down from her leadership role or resign from cabinet.
The Senate will be asked to refer Senator Nash's case to the High Court when parliament returns on September 4.
Federal Labor is expected to have a field day over the latest revelation.
And the frenzy will be compounded by Mr Turnbull heading overseas and Mr Joyce acting in his role running the country while there are still question marks over his eligibility to be in parliament.
If the deputy prime minister doesn't abstain from voting in parliament's lower house when MPs return to Canberra in a fortnight's time, Labor will seek to have all votes deferred until the High Court rules on his eligibility, which could take months.
"I don't believe we can be in a situation where bills are being sponsored by somebody where we don't know whether he's even allowed to legally be a member of parliament," opposition frontbencher Tony Burke told MPs on Thursday.
The prime minister told parliament his deputy was "entitled to be a minister of state under the constitution" for as long he was an MP.
"The reference to the High Court was not done for any reason other than to give the court the opportunity to clarify this area of the law which has been the subject of so much controversy recently," Mr Turnbull said.
Labor leader Bill Shorten said Mr Joyce should not be exercising any responsibilities as a minister.
"I think Barnaby Joyce should do the right thing, let the nation move on from this constitutional crisis that he and his colleagues have embroiled us in," Mr Shorten said.
The government had failed to explain why Senator Canavan has stood aside from cabinet while Mr Joyce had not.
One Nation senator Malcolm Roberts, as well as former Greens senators Larissa Waters and Scott Ludlam, have also been referred to the High Court over dual citizenship.