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, who are expected to have the backing of Labor, believe Senator Roberts' eligibility under section 44 of the constitution could be in doubt because of his British citizenship.
The votes of four crossbenchers will be needed to secure the motion.
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.
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.