Samoa's Supreme Court has ruled that the Fast party's swearing in at Parliament on May 24 was illegal.
But it has also ruled that Parliament must sit within seven 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 court today also warned that any attempt to obstruct the convening of Parliament will be considered contempt of court and Parliament, and would subsequently force the court to validate the invoked principle of necessity "so that the business of the nation can proceed".
The decision from the court follows an application from the Attorney General to stop the legal effect of an impromptu swearing-in ceremony held by Fat on May 24 outside Parliament.
This ceremony was held after Samoa's Speaker of Parliament Leaupepe Toleafoa Fa'afisi, a member of HRPP, prevented MPs-elect from Fast from entering Parliament, despite the Supreme Court's order that Parliament be convened last month.
Also in court this week the caretaker government and officials face accusations of contempt of court for their role in blocking the Fast party from being sworn in.
Fast argues the lockout at Parliament was in defiance of a Supreme Court ruling that Parliament should convene.
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.
Tuila'epa said at least Fast have had the Appeal Courts decision explained to them and they now understood what it meant.
At an evening of singing at HRPP headquarters on Saturday, Tuila'epa 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.
Tuila'epa 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.
Meanwhile, Australia's Government called on Samoa's two political parties to co-operate and convene Parliament, while the Pacific Islands Forum secretary General Henry Puna told media he's been assured by leaders of both parties that they will respect the court's decisions.