Samoa's Supreme Court has ruled that the FAST party's swearing in at parliament on May 24th was illegal.
But it has also ruled that parliament must sit within 7 days, giving FAST the ability to declare government and be sworn in.
Currently FAST has 26 seats, while the caretaker HRPP government has 24.
However due to the political and constitutional impasse since April's general election, parliament has not yet convened.
The Appellate Court on Friday declared that the issue of a contentious sixth women's electoral seat could not prevent the convening of parliament.
That decision refuted HRPP's claim that the extra seat must be appointed before parliament could sit.
Ahead of today's Supreme Court decision, FAST leader Fiame Naomi Mata'afa said her party would continue to push to have parliament convened and for the operational budget to be urgently approved by the end of June deadline.
Fiame has written to the Head of State requesting the house sit on Tuesday.
"If it goes against us, all we'll really need to do is to go back into parliament and get sworn in and just continue to formulate the government based on our numbers."
The Samoa Observer reports the caretaker prime minister - and leader of the Human Rights Protection Party - Tuila'epa Sa'ilele Malielegaoi - has scoffed at FAST's call to convene parliament following the Appellate Court decision.
Tuilaepa said at least FAST have had the Appeal Courts decision explained to them and they now understand what it means.
At an evening of singing at HRPP headquarters on Saturday Tuilaepa said the court had clarified what the decision meant.
"And now they're claiming they won and want Parliament to convene. There's no decision like that," the Observer quotes him as saying.
Tuilaepa maintains that parliament cannot convene until all legal challenges are dealt with and until a sixth woman member has been chosen as per Section 44 of the Constitution.