Labor has called on the Turnbull government to refer Liberal MP John Alexander to the High Court amid mounting evidence he is a dual citizen.
Citizenship experts agree Mr Alexander is likely to be a UK citizen by descent after Fairfax Media revealed his father was born in England before moving to Australia as a young child. Mr Alexander has never renounced UK citizenship and cannot say for sure whether his father did so before he was born.
Experts have also challenged the backbencher's claim that his father had 40 years in which to renounce, confirming the window was actually just two years.
Prime Minister Malcolm Turnbull on Tuesday said Mr Alexander has told the party he "believes" he is not a dual citizen.
But opposition frontbencher Jim Chalmers said Mr Turnbull and his MPs could not be trusted.
"Turnbull needs to explain what it is about Alexander's case that is any different to [Stephen] Parry's, [Barnaby] Joyce's or [Fiona] Nash's," he said. "And if he can't explain the difference, the government must make a referral to the High Court at the next opportunity."
Asked if the government would refer him to the High Court, Mr Turnbull said: "I'm not going to start front-running the process."
But if any MPs believe they are a dual citizen now they should resign, he said.
If Mr Alexander resigns or is found ineligible it would spark a byelection in his Sydney seat of Bennelong. There is speculation that the 67-year-old would not stand again, potentially making it harder for the government to retain the seat.
The former tennis champion is a strong local member and has built his seat's margin by nearly seven per cent since he first won it in 2010. But a new Liberal candidate could make it vulnerable, putting Mr Turnbull's majority at risk.
The claim that Mr Alexander's father may have renounced at some point after he arrived in Australia in 1911 is complicated by the fact the concept of Australian citizenship did not come into force until 1949.
Professor of constitutional law at the University of Sydney, Anne Twomey, said there did not appear to be any legal way for Gilbert Alexander to renounce his citizenship before Australian citizenship came into force.
"Certainly the British Nationality and Status of Aliens Act 1914 did not permit subjects in the self-governing Dominions - including Australians - to do so. It appears that it wasn't until January 1, 1949 that such action could be taken," she said.
World renowned British citizenship expert Philip Gamble, from consultancy Sable International, also believes it is likely Mr Alexander would be a UK citizen.
"Gilbert's status as a citizen of the UK and colonies was not affected by his marriage to an Australian woman nor was he required to renounce British nationality to take Australian citizenship," Mr Gamble said.
"Gilbert and John acquired their British nationality by operation of law and no action was required for them to 'activate' this status - John will be a British citizen by descent unless he has taken the formal step to renounce British nationality."
An online renunciation search tool provided by the British National Archives shows no record of Mr Alexander's father having renounced.
Under the British Nationality Act 1948, people born to British citizens are automatically given citizenship of the UK by descent.
This is the law that conferred British citizenship on former senator Nash and former One Nation senator Malcolm Roberts, who were kicked out of the Senate by the High Court last month. Former Senate president Stephen Parry resigned from Parliament last week after revealing he was a dual UK citizen through the same law.