Maria Lourdes Sereno was the first female chief justice of Philippines' highest court, and she is an outspoken critic of Duterte's controversial war on drugs.
The quo warranto petition filed by Solicitor General Jose C. Calida has been granted by a vote of 8-6.
"If the Chief Justice has been denied of this basic precept that is supposed to be embedded in a democratic process, where justice is to be based on the rule of law, how then can we protect an ordinary litigant who seeks redress before the highest court?" she added. "For the longest time, we have not had a national conversation", she said.
Besides the quo warranto, the en banc, in a separate vote, declared that Sereno violated the Constitution on account of her failure to file her Statements of Assets, Liabilities and Net Worth or SALNs.
Addressing reporters outside the court, spokesman Theodore Te said: 'Sereno is found disqualified from and hereby adjudged guilty of unlawfully holding and exercising the office of the chief justice.
For Bagong Alyansang Makabayan (Bayan), the removal of Sereno is "another act of tyranny that only empowers the Executive, while further undermining whatever is left of the separation of powers of the three co-equal branches of government".
Legal experts pointed out that under the Constitution, all impeachable officials like the president, vice president and chief justice as well as heads of constitutional offices like the Commission on Elections, Office of the Ombudsman and Commission on Audit are to be dismissed only after undergoing an impeachment trial with the Senate sitting as the impeachment court. He said impeachment removes qualified impeachable officers while the other ousts unqualified public officials.More news: SpaceX launches its newest Falcon 9 rocket
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"The Supreme Court has fallen, and fallen hard, in the eyes of the public", she said.
Reyes added that ousting people like Sereno could lead to a situation where "there will be no more balance of power".
Lawyer Mundlyn Misal-Martin, president of the Integrated Bar of the Philippines (IBP) Cebu Chapter, said she was surprised by the decision and that it was "something that I look forward to reading and learning from".
Eight justices voted yesterday to grant the petition and remove the chief justice. He says "the people should express to the court that the decision was wrong and unacceptable". Activists and opposition figures say that corruption allegations against her are a pretext for the real reason she was pushed out: She stood up against the president. He had preempted that the SC will oust Sereno.
Sereno angered Duterte after she disagreed with his efforts to take action against judges authorities linked to illegal drugs in 2016, saying the Supreme Court should be the one to punish erring judges.
"So I'm putting you on notice that I am now your enemy". But there are still some lawmakers calling for the impeachment to proceed independently from the Supreme Court.
Global rights groups and local critics have accused the 73-year-old Duterte of drifting toward authoritarianism after declaring martial law in the south amid a major attack by pro-Islamic State group militants last year.