Turnbull government MP John Alexander is scrambling to confirm whether he is a dual citizen, which if proven would trigger a byelection that would further threaten the Coalition's grip on power.
Fairfax Media can reveal Mr Alexander's father, Gilbert Alexander, was born in the UK and is likely to have conferred citizenship by descent to his son. Mr Alexander has never before been named as an MP with a parent born overseas, meaning his status has flown under the radar since the citizenship saga first erupted in July.
The Liberal member for the Sydney seat of Bennelong has confirmed he never renounced any British citizenship before entering Parliament, and cannot say definitively whether his father did so before the former tennis champion's birth in 1951.
This means he could be ineligible for Parliament under section 44 of the constitution.
A spokesman for Mr Alexander said on Monday: "He believes his dad renounced his UK citizenship in the 41 years he lived in Australia before John was born. But of course he will fully participate in the process that the PM has just announced today."
Prime Minister Malcolm Turnbull on Monday revealed all federal politicians will be required to publicly detail their citizenship history under new transparency measures designed to end the crisis.
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.
That means he may have only had a two-year window to renounce before his son was born in 1951.
Under the online renunciation search tool provided by the British National Archives, there is no record of Mr Alexander's father having renounced at any time after 1948.
Gilbert died in 1987, aged 80.
Fairfax Media has obtained documents that show Gilbert was born in Essex, England in September 1907. He was baptised in London the following month.
An incoming passenger card shows Gilbert and his family travelled from London to Sydney on June 9, 1911, when Gilbert was aged three. He enrolled to vote in the seat of Warringah in 1930 at age 23, and married Mr Alexander's mother, Thelma Fletcher Hipgrave, in 1944. They had John in July 1951.
Under the British Nationality Act 1948, people born to British citizens are automatically given citizenship of the UK by descent.
This law conferred British citizenship on the Nationals' Fiona Nash and One Nation's 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.
Asked if he was confident he would not lose any more Coalition MPs to citizenship issues, Mr Turnbull said on Monday: "The federal director has told me that all of the Liberal Party members believe that they are in compliance with the constitution."
Mr Alexander won Bennelong, a culturally diverse electorate on Sydney's north shore in 2010, defeating sitting Labor MP and former ABC broadcaster Maxine McKew. He was reelected at the 2013 and 2016 federal elections. Mr Alexander holds the seat with a margin of almost 10 per cent.
The Turnbull government is down one lower house seat while former deputy prime minister Barnaby Joyce fights a byelection in his NSW seat of New England.
In a statement issued on Monday night, Mr Alexander's spokesman said: "My father was born in the United Kingdom in 1907, before moving to Australia as a very young child.
"He became an Australian citizen as soon as he could. I understand he renounced his British citizenship before I was born, because he was a proud Australian." i