BARNABY Joyce will be branded an illegitimate understudy if he becomes Acting Prime Minister while Malcolm Turnbull is overseas this week.
It is shaping up as Prime Minister Turnbull's toughest session of Parliament as the Labor Opposition prepares to challenge the legality of his National Party leader, and deputy PM, Mr Joyce.
The High Court is still more than a month away from deciding whether Mr Joyce was eligible to sit in Parliament after the discovery he had dual citizenship, which disqualifies him under the Constitution.
At the end of the week Mr Turnbull will head to a Pacific Island Forum meeting in Samoa and will have to follow the usual practice of leaving the Nationals Leader as Acting PM at home, or nominating someone else.
The post of Acting Prime Minister is usually ceremonial and Mr Turnbull only intends to be away for 24 hours, but Labor will insist there still could be legal consequences if Mr Joyce holds the temporary job.
This week Mr Turnbull will be swamped by a strongly-based legal argument that Mr Joyce is unable to make any decisions without the risk they will later be ruled unconstitutional.
Government Leader in the House of Representatives Christopher Pyne has dismissed Labor's threats.
"The Speaker has ruled that Barnaby Joyce continues as the member for New England until such a time as the High Court rules otherwise," Mr Pyne told the ABC's Insiders on Sunday.
"We're very confident about our legal advice."
However, the Opposition will insist the "rolling circus" of the Turnbull government is unworkable even while Mr Joyce stats in Parliament.
Labor front bencher Tony Burke said "Malcolm Turnbull's refusal to deal with this is holding the country back - this is going to drag on for months.
"Barnaby Joyce should not be made Acting Prime Minister while we don't know if he's even eligible to sit in the Parliament."
Deputy Opposition Leader Tanya Plibersek said "the job of Prime Minister is pretty important, and if the government's argument is that it is only 24 hours that we're going to have this guy acting - and we don't know whether he is eligible to be in the parliament - I don't think that's a very strong argument."
She added: "People need to remember that it is Barnaby Joyce himself and the government itself that has referred (his case) to the High Court," she said.
"While ever a doubt remains over Barnaby Joyce's eligibility to be sitting in the parliament he cannot be the Acting Prime Minister."
Labor will also use the week in Parliament to demand Mr Joyce stand down as Agriculture Minister and Deputy Prime Minister until the High Court rules on his eligibility to sit in Parliament.
Mr Joyce had dual citizenship when elected - he inherited New Zealand citizenship through his father - and could be disqualified by the court when it considers his case next month.
His party deputy, Minister for Regions Fiona Nash, also has an uncertain future before the court because of her dual British citizenship and Labor is demanding she, too, stands down from the front bench.
Labor sources today dismissed suggestions Opposition troops would boycott any parliamentary votes Mr Joyce and Senator Nash took part in, saying the tactic had never been considered.
But Manager of Opposition Business in the House of Representatives, Tony Burke, said of the two Nationals: "Why are they getting special treatment?"
This was a reference to former Resources Minister Matt Canavan, the National who quit cabinet until his own citizenship mix-up - this one related to Italian citizenship - was cleared up.
"Malcolm Turnbull's refusal to deal with this is holding the country back - this is going to drag on for months," he said.
The Prime Minister today accused Labor of wanting to disrupt Parliament rather than deal with the major issues of threats from North Korea and problems with energy prices.
"We are facing on the Korean Peninsular the gravest threat since the Korean War," Mr Turnbull told ABC radio.
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