The Federal Opposition says it is building a case against more Government MPs with citizenship doubts, and is threatening to "go nuclear", as the war of words over the constitutional crisis intensifies.

Key points: Malcolm Turnbull indicates Government can try to refer two Labor MPs to High Court

Malcolm Turnbull indicates Government can try to refer two Labor MPs to High Court Richard Di Natale says Greens will probably back referrals

Richard Di Natale says Greens will probably back referrals Labor frontbench Tony Burke accuses Coalition of desperation

Prime Minister Malcolm Turnbull is warning the Government will refer Labor MPs facing dual citizenship questions to the High Court, accusing the ALP of flouting the law.

Another Coalition parliamentarian was ensnared in the dual citizenship drama on Saturday, with Liberal backbencher John Alexander conceding he was probably a British dual national and resigning from Parliament.

But the Government has been trying to ramp up the pressure on a small group of Labor MPs — including Justine Keay and Susan Lamb — who only moved to renounce dual citizenship just before the last federal election.

Mr Turnbull said it was clear Ms Keay and Ms Lamb were still British citizens when nominations closed last year, and indicated the Government would try to refer both MPs to the High Court when Parliament meets later this month.

"Bill Shorten wants to protect MPs that were and knew they were foreign citizens at the time they nominated," Mr Turnbull said.

"I mean, there is one law — you know what the rule of law means? The rule of law means that it applies to everybody.

"And that means it applies to Government members, crossbenchers and the Labor Party.

"Bill Shorten seems to think the Constitution does not apply to the Labor Party."

The Government's leader in the Lower House, Christopher Pyne, also said the Coalition would "definitely" move to refer Labor MPs with potential dual citizenship.

"What Bill Shorten should do right now is require Susan Lamb and Justine Keay to resign and recontest those seats in by-elections to ensure the Parliament is acting with complete integrity," Mr Pyne said.

The Coalition will need the backing of at least one Lower House crossbencher to refer Labor MPs to the High Court, because both Mr Alexander and former deputy prime minister Barnaby Joyce are contesting their seats in by-elections.

Greens likely to back referrals

Greens leader Richard Di Natale has already said his party would probably back the referrals — which would give the Government the numbers to send them to the High Court.

But a Labor source said that would amount to a declaration of war, and it threatened to retaliate by pursuing other Coalition MPs who may be dual citizens.

"If Turnbull wants to fire this missile, we've got the ammo to go nuclear," the source said.

"If I were Julia Banks, Nola Marino or Alex Hawke, I'd be sweating bullets whenever he talks about referring Labor MPs. He is locking and loading the gun at his own MPs.

"We've been doing our work — the case against these MPs is compelling. We know there are even more Liberals to come out of the woodwork."

Labor frontbencher Tony Burke accused the Coalition of desperation, and suggested it might back away from its threat when Parliament resumes.

"What a desperate born to rule approach... whether that's their view on Monday week I have no idea, and let me tell you I reckon they have no idea either," he said.

The Opposition also pointed out Attorney-General George Brandis warned in August it would be "very dangerous" to refer MPs to the High Court on a party line vote, unless there was "clear evidence" they were dual citizens.

Labor insists Ms Lamb and Ms Keay are in the clear because they took "reasonable steps" to renounce their citizenship before last year's election.

That was the test imposed by the High Court in a 1992 ruling on dual citizenship.

But some constitutional law experts say the High Court's most recent ruling imposes a more demanding test, and it's possible the court may now rule the Labor MPs ineligible because they were still dual nationals when nominations closed last year.