Prime Minister Malcolm Turnbull is continuing to demand that Labor refer its MPs facing dual citizenship questions to the High Court, accusing the Opposition of flouting the law.

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

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.

Justine Keay (left) and Susan Lamb: citizenship issues could be heading for the High Court.

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.

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 the former deputy prime minister Barnaby Joyce are contesting their seats in by-elections.

Meanwhile, Labor frontbencher Tony Burke accused the Coalition of desperation, and suggested the Government 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.

-ABC