Panfilo Lacson, chairman of the Senate foreign affairs committee, said the Commission on Appointments was unhappy with Yasay’s conflicting comments under oath, and their decision was final.

“We all know how close attorney Yasay is to the president. It was the view ... that he was not telling the truth, he was not forthright in the question-and-answer portion of the hearings,” Lacson told reporters.

Yasay, appointed by President Rodrigo Duterte at the start of his administration eight months ago, told an initial hearing that he had never been an American citizen.

Philippine Cabinet ministers must undergo a confirmation hearing, but these often take place long after they begin work.

But on Wednesday Yasay faced the commission again and was grilled over documents that showed he obtained US citizenship in 1986.

The documents also showed Yasay renounced the American citizenship at the US embassy in Manila days before his appointment as foreign secretary.

Watch: former Philippines foreign secretary defends Duterte’s drug war

But Yasay denied being a US citizen, and said that although he was granted citizenship in the 1980s, he was disqualified because he had acquired it with a prior intent to renounce it and return to live in the Philippines.

He said his repeated use of a Philippine passport to visit the United States was “tacit acceptance” by Washington that he was not an American.

However, records of the US Internal Revenue Service suggests Yasay may have held US citizenship when he became foreign minister. The US Federal Register last month published names of individuals who had lost citizenship, whose information had been received “during the final quarter” of 2016.

Yasay’s statements to the committee on Wednesday were loaded with legalese, lengthy and at times, confusing. He apologised to lawmakers and said he had not sought to deceive them.

“I did not lie. I may not have fully disclosed what was required ... but this is really no fun in a process like this,” he said.

“You get nervous. You somehow come up with answers that you do not intend, and I apologise for that.”

Lacson said Yasay could not be reappointed and warned he may face perjury charges.

This would mean the president would have to find a new top diplomat at an important time for the Philippines and while it holds the rotating chair of the Association of Southeast Asian Nations.

Lacson said the decision proved that the commission was not just a rubber stamp.

Foreign affairs spokesman Charles Jose said in a text message the ministry respected the panel’s decision and would ensure a smooth transition as soon as Duterte appointed a new foreign secretary.

Duterte’s spokesman, Ernesto Abella, said the president’s office would name an acting foreign minister on Thursday.

But whoever Duterte names as acting foreign minister would only serve until May.

Duterte is expected to appoint his former running mate in the presidential elections, Senator Peter Cayetano, as permanent foreign secretary. Duterte had promised Cayetano the post shortly after assuming office in June last year. But rules prevent losing candidates taking government posts for a year after the election.

The revelations that Yasay, Duterte’s dormitory roommate at university decades ago, had US citizenship are ironic given the president’s repeated anti-American tirades.

Yasay also regularly criticised the United States as he helped steer Duterte’s foreign policy away from the Philippines’ former colonial ruler and towards China instead.

“We cannot forever be the little brown brothers of America,” Yasay said last year.

“Breaking away from the shackling dependency of the Philippines to effectively address both internal and external security threats has become imperative in putting an end to our nation’s subservience to United States’ interests,” Yasay wrote.

Agence France-Presse, Reuters, Xinhua