Senator Xenophon, whose father came to Australia in 1951 from the then British territory of Cyprus, had been awaiting confirmation from the British Home Office as to whether he was a citizen by descent.
"The circumstances of this are bizarre and rare," he told reporters in Adelaide.
Senator Xenophon will seek leave to refer himself to the High Court, sitting as the Court of Disputed Returns, when parliament resumes in two weeks time.
The parliament has already referred senators Matt Canavan, Larissa Waters, Scott Ludlam and Malcolm Roberts, and Deputy Prime Minister Barnaby Joyce to the court to determine whether they are disqualified under section 44 of the constitution, which bans dual nationals.
Senator Xenophon will continue to vote in the Senate until his case is determined by the court.
British Overseas Citizenship is a historical category described by the British Home Office as "useless" and "a rare peculiarity", he said.
"I never contemplated that I could have been a British colonial citizen, and that is why I didn't go through an act of renunciation with the UK," he said.
"It seems that being born in Australia, according to 1948 UK legislation, makes me a colonial Pom - something that has stunned me and my 86 year old father."
The parliament has already referred senators Matt Canavan, Larissa Waters, Scott Ludlam and Malcolm Roberts, as well as Deputy Prime Minister Barnaby Joyce to the court.
On Friday, Deputy Nationals Leader Fiona Nash revealed she was a UK citizen by descent - the third member of Malcolm Turnbull's cabinet to be affected.
This puts the government's majority of one seat in the House of Representatives at risk should there be an adverse ruling by the High Court, with a decision expected in December.
Of the entire crossbench in the lower house, only Indi MP Cathy McGowan has guaranteed to support government bills ahead of the High Court's consideration of referred cases.
"My primary concern is to support good governance," Ms McGowan said on Saturday.
"This is the basis on which I committed to supporting the government on supply and confidence."
The member for Indi joined other crossbench politicians in the upper and lower houses to call for a full audit of the citizenship status of those in the 45th parliament.
"This proposal may be seen as 'too little, too late', but it would be a positive step towards increasing transparency and compelling all members to reveal their citizenship status," she said.
Speaking to reporters in Melbourne on Saturday, Greens leader Richard Di Natale said it was time Malcolm Turnbull showed leadership and supported the audit.
"We've got the support of every member of the crossbench and yet we've got a Coles and Woolies of politics getting together trying to do everything they can to protect their own patch," Senator Di Natale said.
On Twitter, conservative crossbench senator Cory Bernardi reacted to Senator Xenophon's announcement by repeating his calls for parliament to be prorogued.
"The farce continues. Prorogue parliament now!" he wrote.
