The fate of East Timor's next government could be decided by a judge, with Fretilin arguing it has the constitutional right to appoint the prime minister.
The party refuses to rule out a legal battle should President Jose Ramos Horta challenge its authority.
The constitution states that the president appoints the prime minister, who 'shall be designated by the most-voted political party or alliance of political parties with parliamentary majority'.
Most observers read that as giving Mr Ramos Horta the right to choose the prime minister, in the wake of last week's elections that left no clear winner. They say Mr Ramos Horta is free to pick former resistance leader Xanana Gusmao, whose National Congress for the Reconstruction of East Timor came second at the polls with 23 per cent of the votes.
The congress has since joined forces with three other parties, and controls 37 of the next Parliament's 65 seats.
But legal experts in Fretilin argue the party should be given the mandate as it won 29 per cent of the vote.
'The 'parliamentary majority' in section 106 [of the constitution] does not refer to an absolute majority,' said a statement from lawyers Sahe Da Silva and Jose Teixeira, who are Fretilin members.