The High Court could be asked to find whether One Nation senator Malcolm Roberts is ineligible to sit in parliament.
A Greens motion to refer the Queensland senator to the Court of Disputed Returns is set to be voted on in the Senate on Wednesday afternoon, a day after a coalition senator and two former Greens senators were referred over citizenship issues.
The Greens believe Senator Roberts' eligibility under section 44 of the constitution could be in doubt because of his British citizenship.
Opposition senator Doug Cameron, who also once held British citizenship, indicated the motion would have Labor's support.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"Labor would be inclined to refer Malcolm Roberts' situation to the High Court," he told ABC radio on Wednesday.
The votes of four crossbenchers will be needed to secure the motion.
Liberal Democrat David Leyonhjelm says he has no desire to damage his relationship with One Nation, but could also support the referral.
"On just the facts alone I would support that (motion). But what I'll be doing is listening to the debate," he said.
Senator Roberts and One Nation leader Pauline Hanson have denied on several occasions he ever held British citizenship.
But Buzzfeed has obtained an immigration department document signed by Senator Roberts and dated May 8, 1974, on which the then 19-year-old notes he is a "British UK and Cols (Colonies)" citizen.
The Queensland senator was born in Disergarh, India, in 1955 and his family arrived in Australia in October 1962.
While the documents confirm the Roberts family's notifications to become citizens were approved by the immigration department, there is no mention of whether the British citizenship was renounced.
Senator Roberts has said he took steps to renounce any "possible" British citizenship before he ran for parliament, but did not receive a letter back from British authorities until December last year.
The government won't be supporting the motion as Attorney-General George Brandis says he doesn't believe there is enough evidence to back the referral.
"It is a very serious matter for a group of politicians sitting in a parliamentary chamber to decide to effectively disqualify or put a question mark over the eligibility of one of their number," he said.
The Senate on Tuesday referred former minister Matt Canavan and former Greens senators Larissa Waters and Scott Ludlam to the High Court to test their eligibility.
