Zambia's parliament has delayed a debate due on Wednesday on a motion seeking to impeach President Edgar Lungu over accusations of breaching the constitution, according to a letter from the parliamentary clerk seen by Reuters.

Zambia's main opposition party, the United Party for National Development (UPND), filed a motion last week. The notice set Wednesday as the date for the lawmakers' debate and vote on the motion.

"In view of the gravity of the motion, the same is being studied and we shall revert to you in due course," said the letter sent on Monday from the Clerk of the National Assembly to Garry Nkombo, UPND parliamentary Chief Whip who filed the motion.

UPND spokesperson Charles Kakoma said the motion would now not be debated until parliament's next session begins in June. Signed by a third of the 166-member house, the motion needs the backing of two thirds to succeed.

The opposition argues Lungu violated the constitution in 2016 when it challenged his election victory in court, alleging electoral fraud, a claim the government denied.

The opposition believes that under the constitution, the president should, when faced with a legal challenge of this kind, hand over power to the speaker of the National Assembly until the court makes a decision.

The UPND's case against Lungu was dismissed on a technicality after its lawyers failed to provide evidence within the required time limit.

Chishimba Kambwili, a lawmaker of Lungu's ruling Patriotic Front who seconded Nkombo's motion, was arrested last Thursday by Zambia's Anti-Corruption Commission on suspicion of fraud.

Kambwili, a former cabinet minister who has repeatedly accused government officials of corruption, fell ill while in police custody and was taken to a private hospital for treatment, police and his lawyers said.

He was subsequently transferred to a government hospital for continued treatment on Tuesday, they said.