One Nation Senator Malcolm Roberts was a citizen of the United Kingdom by descent at the time of his nomination, the High Court has found.

Key points: Malcolm Roberts knew he did not become an Australian citizen until 1974, Justice Patrick Keane says

Malcolm Roberts knew he did not become an Australian citizen until 1974, Justice Patrick Keane says Senator Roberts concedes he never read citizenship form he signed

Senator Roberts concedes he never read citizenship form he signed One Nation senator in a more "vulnerable" position, expert says

Justice Patrick Keane said in his findings that Senator Roberts knew he did not become an Australian citizen until 1974.

"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," he wrote.

It came after Senator Roberts was cross-examined for about three hours on Thursday, rejecting suggestions he knew of his British citizenship before nominating for Parliament.

Senator Roberts said he did not read an Australian citizenship form before signing it when he was 19 years old, which contained a line confirming he was a UK and Colonies citizen.

He said his then-16-year-old sister filled out the form.

He also maintained that he believed he was "Australian, and only Australian" when he tried to send an email to British authorities with the subject line "Am I still a British citizen?" before the election.

After the testimony, Senator Roberts' own lawyer conceded his client's evidence may have affected his credibility.

The full bench of the High Court will determine if Senator Roberts breached Section 44 of the constitution. ( AAP: Mick Tsikas )

Roberts Newlinds said it was "inevitable" the court would find Senator Roberts had some knowledge that he was British in the lead-up to becoming an Australian citizen in 1974.

"He's talked himself into a different belief about that," Mr Newlinds told the court.

Justice Keane found the email "was not apt to lead to a termination of his British citizenship because it did not contain a declaration of truth".

"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," he wrote.

"I am not persuaded that Senator Roberts was not sincere in his tenacious advocacy of the position to which he committed himself by signing his nomination to the Senate on 3 June 2016; but the difficulties in his position to which I have referred test credulity too far to accept his evidence that he was confident that he was not a British citizen at the date of his nomination."

It will be up to the full bench of the High Court to determine if Senator Roberts breached Section 44 of the Constitution by having dual citizenship, and not taking "reasonable steps" to renounce it.

Roberts in 'more vulnerable position' than others in court

University of Sydney constitutional law expert Professor Anne Twomey said the One Nation senator was now in a more "vulnerable" position in his legal battle to remain in the Senate than other parliamentarians referred to the High Court.

She said the case could rest on his knowledge of his dual citizenship.

"[Justice Keane's ruling is] attributing to him a degree of knowledge that would take him outside of the argument that the Commonwealth is putting," she said.

"So that puts him in a more vulnerable position than the other senators and members that the Commonwealth asserts is protected by that argument.

"If the test is 'have you taken all reasonable steps', the other thing that comes out in Justice Keane's decision is the steps that Senator Roberts took were not terribly effective."

If Senator Roberts' election is found to be invalid, it will be up to the court to determine the method of replacing him in the Senate.

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