The High Court has found that One Nation Senator Malcolm Roberts was a British citizen by descent at the time of his nomination.
Justice Patrick Keane said that Roberts "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."
"Senator Roberts could have made effective inquiries of the British High Commission by which he would have been informed of the steps necessary to renounce his foreign citizenship," Keane found.
Roberts, who was born in India to a Welsh father, came under scrutiny over whether he was a citizen of any other country at the time of the July 2016 election.
The court found that the only step "arguably" taken by Roberts to renounce his citizenship before his nomination was an email sent on the 6 June 2016.
"This email could not be effective as a renunciation of his UK citizenship because it did not contain a declaration of truth, and it was not accompanied by a prescribed fee," Keane said.
Roberts is one of seven politicians currently involved in a dual citizenship scandal that could have ramifications for his place in the Federal Parliament.
As reported by the ABC, a full bench of the High Court will now determine if Roberts has breached Section 44 of the Constitution, which would render him ineligible to serve in the Parliament.
If it is found that Roberts did not take reasonable steps to renounce his British citizenship, thereby invalidating his election, the court will have to determine the method of replacing him in the Senate.