There was no immediate comment from Sirisena or Rajapakse after the Supreme Court's unanimous decision
Sri Lanka's Supreme Court on Tuesday overruled President Maithripala Sirisena's dissolution of parliament in a major boost to sacked prime minister Ranil Wickremesinghe's fight to reclaim the post from an arch-rival.
The heavily guarded court also halted preparations for a snap election in the latest twist in a power struggle that began when Sirisena dismissed Wickremesinghe on October 26 and replaced him with former strongman president Mahinda Rajapakse.
The 225-member parliament could meet as early as Wednesday to decide which of the duelling pair it backs.
Amid mounting international concern, Wickremesinghe has refused to accept his sacking and has remained in his official residence while Rajapakse has sought to build a parallel administration.
Wickremesinghe's United National Party (UNP) is the largest single party in the assembly. He told reporters that he expected the legislature to meet Wednesday, as was scheduled before Sirisena dissolved the body to prevent a majority test.
"I will go to parliament tomorrow and we will show we are the legitimate government of Sri Lanka," Wickremesinghe said at the Temple Trees residence that has become the opposition headquarters.
Wickremesinghe called the court ruling a "a victory for the people and a victory for decent politics in this country."
Parliamentary officials said Speaker Karu Jayasuriya called a meeting of political party leaders on Wednesday morning ahead of a formal meeting of legislators who could vote between Wickremesinghe and Rajapakse.
There was no immediate comment from Sirisena or Rajapakse after the Supreme Court's unanimous decision.
The UNP led petitions against the dissolution of parliament and the three-judge bench, headed by the Chief Justice Nalin Perera, gave the landmark decision to a packed court guarded by hundreds of heavily armed police and commandos.
The court said it would give a final verdict on the petition on December 7, after three more days of hearings.
The judges ordered the independent Elections Commission to halt preparations for the January 5 vote, which Sirisena had announced on Friday.