Federal Labor MP David Feeney has resigned from parliament after being unable to confirm if he is a dual citizen.
Mr Feeney said he had written to the Speaker of the House of Representatives on Thursday and his resignation was effective immediately.
He said he did not have sufficient paperwork to prove he had renounced his British citizenship a decade ago, inherited through his Northern Irish father.
"I am unable to disprove that I am a dual citizen," Mr Feeney, who held the marginal Victorian seat of Batman, said during a press conference on Thursday.
"I have spoken to my family and I have decided that I will not be seeking ALP preselection for this by-election.
"This has been an incredibly difficult decision but both my community and the Labor Party I love deserve a candidate that is able to give the months and the years ahead 150 per cent of their effort, their commitment and their passion.
"After careful reflection, I don't believe I'm able to offer this. That tells me that it's time for me to stand aside for a Labor candidate that can and will. It has been an honour for me to serve."
Following Mr Feeney's resignation, Defence Industry Minister Christopher Pyne said this had been a "day of complete humiliation for Bill Shorten".
"We've seen David Feeney dragged, kicking and screaming, to resign from the House of Representatives when he found, to use his own words, that he couldn't disprove that he was a citizen of the UK," Mr Pyne said.
Mr Pyne also called on Labor MP Susan Lamb, who has also been caught up in the dual citizenship saga, to resign as she is still yet to discover if she holds UK citizenship.
"The advice from her own barrister indicates she's never relinquished her UK citizenship," Mr Pyne said.
"Bill Shorten now faces the prospect that he must cause Susan Lamb to resign from the House of Representatives so that by-elections for Longman and Batman can be held on the same day, saving the Australian taxpayer money rather than having separate long, drawn-out by-elections and another High Court referral of Susan Lamb to the High Court, costing taxpayers money yet again."
One of the possible replacements being flagged for Mr Feeney is ACTU president Ged Kearney, who considered running for preselection in the seat in 2013.
The Australian Greens have already selected Alex Bhathal as a candidate.
However, she is reportedly the subject of an internal complaint by party members over her lack of support for local Greens councillors.
- with AAP
