The High Court has ruled One Nation senator Malcolm Roberts was a dual British and Australian citizen when he nominated as an election candidate.
Justice Patrick Keane on Friday ruled the Queensland senator, who believed he had renounced his British citizenship before last year's election, was a citizen of the UK at the time.
Born to an Australian mother and Welsh father in India in 1955, he is the first of seven parliamentarians who will have their futures considered by the High Court because the constitution bans dual citizens sitting in parliament.
Senator Roberts' matter will now go before a Full Court hearing on October 10.
However, Friday's decision by Justice Keane, who found Senator Roberts was likely aware he was a dual citizen, casts grave doubt over his eligibility to continue in the Senate.
If disqualified, his Queensland seat would pass to the third person on the One Nation 2016 ticket, Fraser Anning.
Mr Anning, however, is the subject of bankruptcy proceedings before the Federal Court and bankrupts are also precluded from sitting in parliament.
The fourth person on the ticket is One Nation Leader Pauline Hanson's sister, Judy Smith.
Senator Roberts told the High Court on Thursday he believed he renounced his foreign citizenship but also admitted he knew there was a possibility he was a British or Indian citizen when he ran for federal parliament.
"I couldn't be absolutely certain, even though I felt certain," he testified.
"I had very strong confidence that I was Australian and only Australian. That's the way I was raised."
Senator Roberts said he had "no hesitation" when signing a form on 29 April 2016 to nominate for One Nation but later inquired about his citizenship in emails intended for the British consulate.
Justice Keane ruled Senator Roberts' efforts to renounce his UK citizenship through further emails was "not apt to lead to a termination ... because it did not contain a declaration of truth".
He also found it could not lead to a termination because it was "not accompanied by the prescribed fee".
"I find that Senator Roberts knew that he did not become an Australian citizen until May 1974," Justice Keane said in his judgment.
"I find that, as at the date of his nomination for the Senate, he knew that there was at least a real and substantial prospect that prior to May 1974 he had been and remained thereafter a citizen of the United Kingdom."
Justice Keane said Senator Roberts should have made more effective inquiries with the British High Commission which would have informed him of the steps necessary to renounce his foreign status.
One Nation and Senator Roberts have been contacted for comment.