One Nation Senator Malcolm Roberts has revealed he did not fill out a form renouncing his British citizenship until after his election nomination.

Senator Roberts, who previously claimed he had the necessary documents to be eligible to stand at the last election, has now admitted he did not send an official letter to the UK until after the nomination date.

“It was after the nomination date but the key point is that I asked the British to point out whether I am a British citizen and they couldn’t do it,” he told Sky News.

“So I said I am renouncing (my citizenship) just in case.”

The revelation puts Mr Roberts most at risk of being disqualified from parliament when the dual citizenship debacle fronts the High Court for a three-day hearing in Canberra from October 10.

Mr Roberts is one of four current and former Senators – including deputy prime minister Barnaby Joyce – referred to the court earlier this month after it was revealed they did not meet the sole citizenship requirement of section 44 of the constitution.

But despite the declaration, Mr Roberts remains confident the High Court will rule in his favour.

“The facts will stand and we are very happy to produce those facts in the High Court,” he said.

“I think what you’ll find is that I actually got off my butt and actually researched it, even thought I didn’t actually think I was British or Indian.

“I don’t see evidence that the others have done any research so I think I have a very good case.”

South Australian Senator Nick Xenophon and NSW Senator Fiona Nash will also be referred to the court over their British citizenship by descent when federal parliament next sits in September.

Solicitor-General Stephen Donaghue QC said there was a “clear demarcation line” between the politicians – those who had known they were a citizen of a foreign power and those who did not.

Under section 44 of the constitution a person is incapable of being elected to the parliament if they are a “citizen or entitled to the rights or privileges of a subject or a citizen of a foreign power”.

Mr Joyce has been found to have been a New Zealand citizen by descent but the government has advice this isn’t enough to unseat him.

If the court finds him to be ineligible, it will trigger a by-election in his NSW seat of New England, and potentially put at risk the government’s one-seat majority in the lower house.

The court will also examine stood-aside cabinet minister Matt Canavan, who has Italian heritage on his mother’s side.

Like Senator Roberts, former Greens senators Larissa Waters and Scott Ludlam were also born overseas.

– with AAP