MANILA- Four head revisors tasked to manually recount the votes in the 2016 vice presidential race have resigned a day after the Presidential Electoral Tribunal (PET) began the process.

They were among 40 head revisors supposed to verify the physical count of the ballots for the election protest of former Sen. Ferdinand "Bongbong" Marcos Jr. against Vice-President Leni Robredo.

• Leni vs Bongbong: VP vote recount starts

In a statement, Marcos said he was "surprised and concerned" with the resignation of the revisors allegedly "for no apparent reason."

"I hope this will not result in another round of delays especially now that we have started to uncover clear signs of fraud," he said.

"They are no ordinary revisors, having undergone rigid psychological test and meticulous screening by the PET. They must have a compelling reason for backing out and I am one with the Filipino people in asking why," he added.

Beng Sardillo, a lawyer for Robredo, meanwhile, lamented the delay in the recount that the resignation has caused.

"It is unfortunate as this will once more cause delay in the proceedings," Sardillo said.

The vote recount Marcos sought will first cover the pilot areas of his protest: the provinces of Camarines Sur, Negros Oriental and Iloilo, all known bailiwicks of Robredo, the PET had said.

Robredo has a pending counter-protest against Marcos, also before the PET. It covers 31,278 precincts, while Marcos’ protest covers 132,446 precincts.

It was the first time that PET held an actual recount of votes since the body was created by the 1987 Constitution to tackle electoral protests in the presidential and vice presidential races.

Marcos lost to Robredo by 263,473 votes, the closest vice presidential race in recent memory.

• Marcos: Some ballots wet, audit logs missing in Robredo stronghold
• Robredo says recount a 'fight for the truth'