Former President Rodrigo Duterte's anticipated absence from the House Quad Committee's hearing on his drug war, while planning to attend a similar inquiry in the Senate, would be a "slap in the face" to congressmen, panel co-chair Dan Fernandez said Wednesday.

Duterte skipped the quad committee's ninth investigation on Tuesday, citing illness. His legal counsel, Martin Delgra III, assured lawmakers that Duterte remains keen to participate in the probe and will appear before the committee in November.

Despite missing the quad committee session, Duterte is expected to attend the Senate's first hearing on his controversial anti-drug campaign scheduled for Monday.

Fernandez expressed that it would be "unfair" and "indecent" if Duterte misses the quad committee again but appears before the Senate.

"It would be a slap in the face of [the House] if he doesn't attend and only shows up in the Senate," Fernandez said during a briefing.

"He has the right as the former president, but he needs to explain his reasons to us. Is it because we are adverse and [senators] are more lenient?" he questioned.

Fernandez warned that the committee would be forced to "act accordingly" if Duterte's absence from the House continues, describing it as an "outright insult."

"I want to accord the [former] president the highest respect unless he attends the Senate and skips the [House]. That's a different issue," he emphasized.

However, quad committee chairperson Ace Barbers asserted that he would not issue a show cause order or subpoena to Duterte out of respect for his former position.

"Stating for the record yesterday, we should consider one, his age, and two, his being the former president as perhaps an exception to the rule because not all we invite and issue show cause orders or subpoenas to are former presidents," Barbers noted.

"That's why in deference to him being a former president we will perhaps implement exceptions."

The quad committee formally invited Duterte last week after his name was repeatedly implicated in alleged extrajudicial killings (EJKs) during his administration.

This includes the murder of three Chinese drug lords at the Davao Prison and Penal Farm in August 2016, just two months after Duterte took office.

Duterte's former trusted aide, retired police colonel Royina Garma, claimed that the ex-president ordered the police to replicate the "Davao model" nationwide, offering rewards of up to P1 million for killing drug suspects.

Senator Ronald "Bato" Dela Rosa, a close ally of Duterte and former police chief who led the drug war, confirmed that Duterte will attend the Senate hearing next week.