Liberal MP John Alexander has resigned from parliament amid growing speculation he holds British citizenship, describing the Australian constitutional crisis as an "absurd situation".
The member for Bennelong announced his resignation from parliament on Saturday, saying he most likely holds UK citizenship.
Mr Alexander announced that he'll stand again for parliament in an upcoming by-election after renouncing any foreign citizenship.
Earlier this week, Mr Alexander, sought advice on whether he is a UK citizen through his father Gilbert Alexander, who was born in England in 1907 and arrived in Australia in 1911.
Mr Alexander said he hadn't received official confirmation of his dual citizenship, but said the "balance, the probability of evidence is that I most likely am".
"The obligation that I have is that once I do not hold the view that I'm solely Australian I must resign," Mr Alexander told reporters in Sydney.
"Having determined that late yesterday, to my satisfaction, just my choice, I felt I had to resign, that's why I have chosen to do."
Prime Minister Malcolm Turnbull, who spoke to Mr Alexander on Friday night on the telephone, said he had done the "honourable thing".
"John's done the right thing, the honourable thing, he has resigned his seat," Mr Turnbull told reporters in Vietnam.
Mr Alexander said the Prime Minister had told him to act quickly and decisively during the phone call.
"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," he said.
Labor MP Tony Burke confirmed that the party would run a candidate in Bennelong, which the Liberal's hold with a nine per cent margin.
"We don't pretend for a moment that we'll be anything other than the underdog in the by-election, but Labor will be contesting," Mr Burke told reporters.
"It will be a tough campaign but it's one that we'll certainly be in."
He will be the second member of the House of Representatives forced to resign because of their dual citizenship.
Barnaby Joyce is facing a by-election in New England, after discovering he was a New Zealand citizen, because of his father's birth.