Prime Minister Malcolm Turnbull has threatened to break with longstanding precedent and use the government's slim majority to refer any Labor MPs under a citizenship cloud to the High Court.
The Coalition has until now staunchly insisted that any High Court referrals must be made by an MP's own party, as part of a bid to prevent an outbreak of partisan referrals.
In a sign of escalating tensions between Mr Turnbull and Opposition Leader Bill Shorten as they try to negotiate a fix to the citizenship crisis, the pair exchanged strongly-worded letters of demand and counter-demand on Thursday.
With three Labor MPs now in the frame - Justine Keay, Susan Lamb and Josh Wilson - Mr Shorten sought an assurance from Mr Turnbull that "all parliamentarians will abide by the established precedent that there should be no partisan referrals".
"Accordingly no party should abuse its numbers, in either chamber of the Parliament, to unilaterally refer senators or members that have stated the clear grounds on which they are eligible under section 44 and are consistent with the most recent decisions of the High Court," Mr Shorten said.
But Mr Turnbull fired back soon after.
"I cannot give you that assurance," Mr Turnbull said in his letter, which he wrote on the plane en route to Vietnam for APEC.
"The government will vote to refer any individual to the High Court if there are substantial grounds for believing they are in breach of the constitution."
Just a few months ago, Attorney-General George Brandis said High Court referrals "should never be done on a party-line vote", calling it "a very dangerous course".
The pair met for two hours on Wednesday to try and thrash out a compromise agreement to Mr Turnbull's new parliamentary disclosure regime, designed to end the crisis that has consumed the Parliament for nearly five months.
Mr Shorten says he wants disclosures made by December 1, to give the Parliament time to refer any suspect MPs to the High Court before finishing for the year. Mr Turnbull now proposes December 7 - the last scheduled sitting day - which would still require Parliament to be recalled the following week to make any necessary referrals.
Ms Keay, Ms Lamb and Mr Wilson are now considered at threat because they had not received confirmation of their renunciation until after they nominated as candidates in June last year. Under a black-letter reading of the High Court's decision last month, that could mean they too are ineligible to remain in Parliament.
Lower house crossbencher Rebekha Sharkie is also believed to be in a similar situation. Prominent constitutional experts have said they believe the court should rule on their cases if only to achieve further clarity.
Former deputy PM Barnaby Joyce, who is fighting a byelection in his NSW seat after being ruled ineligible, told Leigh Sales on 7.30 on Thursday evening that he was frustrated by MPs who hadn't come forward before the High Court ruling.
"At the start the National Party put themselves out, obviously in my case with the help of Penny Wong's staffers sneaking around in New Zealand," he said.
"But we 'fessed up and myself, Matty Canavan and Fiona Nash put ourselves before the High Court, two of us now don't have a job."
He said any other MPs in similar circumstances should have done the same thing.
"Maybe the High Court if they'd seem them show up as a job lot might have had a different perspective on how big this issue is."
Mr Joyce also took aim at Mr Shorten.
"You can't say you're holier than thou and then not actually go through the process of proving that holiness you purport to have," he said.
In his letter, Mr Shorten also said he wants MPs to publicly disclose all issues that could see them ruled ineligible from Parliament - such as criminal convictions, bankruptcy proceedings and government contracts - not just their citizenship status.
Junior government minister David Gillespie is already before the High Court for a potential government contract, and questions have also been raised about Queensland businessman turned backbencher Barry O'Sullivan.
Mr Shorten also said any ministers who come under a citizenship cloud must immediately be stood down - unlike Barnaby Joyce and Fiona Nash. He also called for Communications Minister Mitch Fifield to be referred to Parliament's powerful Privileges Committee for concealing what he knew about former Senate president Stephen Parry, who resigned from Parliament last week after confirming he was a dual UK citizen.
But Mr Turnbull dismissed this suggestion as "absurd."
Mr Shorten said he had also rejected a suggestion by Mr Turnbull that with the exception of same-sex marriage the Parliament only deal with non-controversial legislation for the rest of 2017.
Tasmanian crossbencher Jacqui Lambie meanwhile has said she will resign if documents she's seeking fail to prove her belief that she is not a UK citizen by descent, a status she may have inherited from her Scottish father.
"Four and a half years ago when we sat down and looked at all this and we had advice and everything was tickety-boo and they said you don't need to renounce your citizenship because you have nothing to bloody renounce," she told Tasmania Times.
"If that be the case and the checks are done and they are not satisfied with what's going on, then I will have to resign. Simple as that."
Liberal MP John Alexander has said he is is still waiting on advice from the UK authorities about his status.
