Turnbull government MP John Alexander has resigned from Parliament after British authorities were unable to uncover any evidence to support repeated assurances he was not a dual citizen.
Mr Alexander's resignation - five days after Fairfax Media revealed he was likely a dual citizen by descent - will trigger a byelection in the former tennis champion's Sydney seat of Bennelong, and means Prime Minister Malcolm Turnbull has lost his parliamentary majority.
"I have always believed that I am Australian and solely Australian," Mr Alexander said.
The 67-year-old told Mr Turnbull he intended to resign during a "business-like" phone call about 6pm yesterday.
"He encouraged me to be decisive, to act expeditiously, and to put things beyond any question of doubt and that's what I think the people of Bennelong want," Mr Alexander said. "Australia is tired of this absurd situation. I don't have any degrees, I have a degree in common sense and it doesn't make any common sense."
The absence of Mr Alexander and former deputy prime minister Barnaby Joyce sees the government reduced to just 74 of the 150 seats on the floor of the House of Representatives during the final two sitting weeks of the year. While the government will not fall, Labor and the crossbench could combine forces to cause trouble when Parliament resumes on November 27.
Asked whether a general election was looming, Mr Turnbull on Saturday responded with a simple "no".
Mr Alexander held the culturally diverse electorate of Bennelong with a margin of 10 per cent, which is normally considered safe. But some Liberal Party figures in NSW fear the seat is vulnerable despite Mr Alexander's strong personal support. Labor held the seat from 2007 to 2010, when former ABC star Maxine McKew snatched it from former prime minister John Howard.
"In any competition, you've got to keep an open mind there is a chance of losing," Mr Alexander said.
Mr Alexander is a British citizen by descent because his father, Gilbert Alexander, was born in England before moving to Australia as a young child.
When Fairfax Media unearthed documents proving his British connection, the Bennelong MP admitted he had never renounced UK citizenship and for days could not say for sure whether his father did, but repeatedly assured the public and the Liberal Party that was his "belief".
He needed to quit no later than Monday to allow the minimum 33-day campaign required to hold the poll on December 16.
The weeks after that date - during the Christmas and New Year period,- would be impractical, pushing back the next likely date until at least early February.
A complication is that Mr Alexander would have to fully renounce his claim to UK citizenship before nominating as a candidate. If he failed to have written confirmation of his status before standing he could still be considered constitutionally ineligible.
Prime Minister Malcolm Turnbull has said MPs who have no doubt that they are dual citizens should resign from Parliament immediately.
On Saturday he called on Labor MPs who were possible dual citizens to also resign so a series of byelections can be held on the same date.
Labor's candidate for Bennelong at the 2016 election, Lyndal Howison, accused Mr Alexander of disrespecting the constitution and Parliament.
"His father was born in the UK and this can hardly have escaped his attention during recent weeks, when his parliamentary colleagues were embroiled in one of the greatest political crises of recent years," Ms Howison said.
"Yet he stayed silent. When finally exposed, he invoked a response reminiscent of that famous scene in The Castle: 'It's the vibe of the thing'. It's one thing to believe you are Australian, it's another thing to have shown a troubling lack of attention to the preparation required to stand for Parliament."