CANBERRA, Australia (AP) — All Australian senators have three weeks to prove they were not foreign nationals when elected under an agreement the major political parties reached Monday to resolve a deepening citizenship crisis that could upend the government.

Prime Minister Malcolm Turnbull's conservative coalition could lose two seats at by-elections next month after government lawmaker John Alexander on Saturday resigned from Parliament because he had likely inherited British citizenship from his English-born father.

Australia is rare if not unique in the world in banning dual nationals from sitting in Parliament. Pressure is growing to reform the 116-year-old constitution amid the growing uncertainty over how many by-elections might result from the current crisis and which party might end up forming government.

Turnbull's conservative Liberal Party and the center-left opposition Labor Party agreed to set a Dec. 1 deadline for senators to provide documented evidence that they are solely Australian citizens. Australian-born lawmakers will have to provide details of their parents and grandparents' dates and countries of birth to demonstrate that they have not inherited a second nationality. Immigrant lawmakers must document steps they have taken to renounce their original nationalities.

The bipartisan support ensures the Senate will endorse that agreement later Monday.

Acting Prime Minister Julie Bishop said she expected the House of Representatives would endorse a similar citizenship registry when it next sits on Nov. 27.

Having lost two seats and its majority in the House due to the citizenship crisis, the government could need the support of a single lawmaker from among the opposition and five independent legislators to get the House's endorsement of such a register.

Turnbull said he would have preferred if the High Court had accepted the government's argument last month that ignorance of a second nationality should be an excuse for breaching a ban on dual citizens that was designed to exclude lawmakers with divided loyalties.

"I didn't seek the decision of the High Court. I would've preferred they took a different approach," Turnbull told reporters in the Philippines where he attended a Pacific Rim leaders summit.

"But what they say is the law and our job now is to comply with it and what we are doing is going through a process that will enable everybody to put all the relevant facts on the line," he added.

Any lawmakers who remain under a cloud after declaring their citizenship status would be referred to the High Court to decide whether they were legally elected. A series of by-elections that could change the government could be scheduled for a single weekend early next year.

Earlier Monday, the government said it would invite the High Court to disqualify at least two opposition lawmakers from Parliament if they did not follow Alexander's example by quitting over questions about whether they renounced British citizenship in time to legally run for election last year.

The minor Greens party had offered to provide the single vote the government would need to get that referral through the House in the face of Labor opposition.

The nine lawmakers who have been scrutinized or referred to the court since July have acknowledged questions about their citizenship needed to be resolved by the court. Most were ruled ineligible.

The citizenship crisis has been blamed for Turnbull's popularity falling in an opinion poll published in The Australian newspaper on Monday.

The High Court last month disqualified Deputy Prime Minister Barnaby Joyce because he had inherited the citizenship of his New Zealand father. He immediately renounced his New Zealand citizenship and will contest his seat at a by-election on Dec. 2.

Alexander also plans to renounce any British citizenship and run for his Sydney-based seat on Dec. 16.

The dual citizenship ban was a rare issue until recently, but the High Court last month disqualified five lawmakers, including Joyce, in a rejection of the government's argument that ignorance of an inherited nationality was an acceptable excuse.

Senators are usually replaced from the same party without elections, but most crucial are the fates of lawmakers in the House of Representatives, where parties need a majority to form a government.

Many argue that the dual citizen ban is increasingly inappropriate for a migrant nation where half the population is an immigrant or has an immigrant parent. But changing the constitution requires all registered voters to cast ballots in a referendum, which rarely succeed.