Duterte issued a withdrawal notice in March, saying his right to due process was violated after an ICC prosecutor announced an inquiry into a complaint that accused the president of crimes against humanity.
A minority bloc in the Philippine senate on Wednesday asked the Supreme Court to invalidate President Rodrigo Duterte's withdrawal from the International Criminal Court (ICC), saying his action without the chamber's consent was unconstitutional.
Duterte issued a notice of withdrawal in March, saying his right to due process had been violated after an ICC prosecutor announced a preliminary examination of a complaint that accused the president and top officials of crimes against humanity.
But six minority members of the senate, from among a total of 23, said Duterte would need the backing of two-thirds of upper house members to get his way.
"The executive cannot unilaterally withdraw from a treaty or international agreement, because such withdrawal is equivalent to a repeal of a law," the senators told the Supreme Court.
Duterte's spokesman, Harry Roque, said he was confident the Supreme Court would throw out the petition.
"Good luck to them," he said. "The courts will always defer to the executive on matters of foreign affairs."
The complaint to the ICC, filed by a Philippine lawyer and endorsed by a congressman and a senator, concerns the deaths of thousands of Filipinos in Duterte's signature crackdown on illegal drugs. Duterte denies any wrongdoing.
Police reject allegations by activists of systematic murder and cover-ups during a bloody campaign that has alarmed the West and gained the maverick Duterte international notoriety.
Critics have dismissed as cowardly Duterte's decision to turn his back on the ICC, calling it contrary to his stated readiness to "rot in jail" if it meant saving his nation from the scourge of drugs.
Lawyers and jurist groups say the withdrawal is pointless and does not protect Duterte against a possible indictment, as the ICC's jurisdiction retroactively covers the period during which a country was a member of the court.