The ruling is a major blow to Sirisena, seven weeks into a major political crisis in the Indian Ocean island nation that has sparked alarm abroad and concerns about its finances.
![Begging migrant children are treated particularly cruelly [Vecteezy]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Begging migrant children are treated particularly cruelly [Vecteezy]")
The seven-judge bench unanimously decided that Sirisena violated the constitution when he dissolved parliament last month and called a snap election nearly two years ahead of schedule.
Delivering the landmark judgement to a packed courtroom, Chief Justice Nalin Perera said the judges agreed that Sirisena violated the constitution when he dissolved the legislature prematurely.
Sacked prime minister Ranil Wickremesinghe's party has said it would await the outcome of Thursday's decision before deciding whether to open impeachment proceedings.
Sirisena triggered the unprecedented political crisis on October 26 when he fired Wickremesinghe and appointed the contentious former strongman Mahinda Rajapakse in his place.
Sirisena then dissolved parliament on November 9 despite provisions that he could not dissolve the legislature until it completes four and a half years out of its five-year term which ends in August 2020.
Four days later after parliament was sacked, the Supreme Court issued an interim ruling suspending Sirisena's decree and restoring parliament, which almost immediately passed a no-confidence motion against Rajapakse.
Wickremesinghe's party and their allies command a majority in the 225-member assembly.
On Wednesday, the legislature voted overwhelmingly to demand the reinstatement of Wickremesinghe as the power struggle headed for a government shut down within weeks.
Members of Wickremesinghe's party and their allies voted 117-0 asking Sirisena to restore the status quo prior to October 26 when he dismissed his former ally.
It was not immediately clear if Wednesday's resolution would be accepted by Sirisena who has vowed he will not give the job back to Wickremesinghe.
Courts have also prevented Rajapakse and his disputed cabinet exercising the powers they claim until they can prove their legitimacy. A hearing by the Court of Appeal on Wednesday was put off until January 16.
Former finance minister Ravi Karunanayake said the entire public sector will come to a complete standstill from January 1 in the absence of a budget for the New Year.
Officials have expressed similar fears and urged Sirisena to resolve the crisis at the earliest. Sri Lanka's credit ratings have already been cut.
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