Estrada, chairman of the foreign relations committee of the Commission on Appointments, said he would move for the UN to reject the appointment of Davide for lack of confirmation from the CA.
The opposition lawmaker said the oath-taking of Davide at the sidelines of the recently concluded 12th ASEAN Summit disregarded Congress.
"The UN cannot accept an illegitimate ambassador who has not yet been confirmed by the Philippine Congress," he said.
Estrada earlier blocked the confirmation of Davide as ambassador to the UN before the CA last year.
Davide was questioned over his decision to administer the oath of office to then Vice President Gloria Arroyo who succeeded to the presidency following the ouster of Joseph Estrada in January 2001.
Estrada echoed the view of his mother Sen. Luisa Ejercito, pointing out the reputation of the Philippines would be put in question if Davide assumes his post at the UN even before his confirmation at the CA.
"Mrs. Arroyo had again committed a diplomatic blunder by insisting that Davide assume his post in New York even without the required CA confirmation," Estrada said.
"An ambassador cannot assume any diplomatic post without going through the legal process, and Davide who is a former chief justice should know what is legal and what is not," he added.
Former senator Francisco Tatad also pointed out Davide, who is already 70 years old, is well over the age limit for diplomats under the law.
Davide, for his part, had explained it is the prerogative of the President to appoint whoever she wants regardless of age.
On Monday, Ejercito branded Davide’s oath-taking as a clear payback by the President for an overdue political debt.
But Executive Secretary Eduardo Ermita said he saw nothing wrong in Mrs. Arroyo’s swearing in Davide pending his confirmation by the CA.
He said there was no legal impediment to Davide’s being sworn in as he was reappointed and his papers submitted to the CA for confirmation. "He got a new appointment so that’s the basis."
"The President can always be a subject of concern or question of people not necessarily in the same political camp as the President but there’s nothing illegal about it. He’s very qualified," Ermita said. — Marvin Sy