The Democratic Alliance (DA) believes the resignation of Investigating Directorate Against Corruption (Idac) head Advocate Andrea Johnson is a direct result of mounting public scrutiny over her leadership.
President Cyril Ramaphosa has accepted Johnson's resignation after she requested to waive her three-month notice period and step down within 24 hours.
Her resignation follows her appearance before the Madlanga Commission of Inquiry last week, where her testimony came under intense scrutiny. Johnson is expected to return to the commission in Pretoria on Monday to continue giving evidence.
DA Chief Whip Glynnis Breytenbach said the resignation appeared to be the outcome of sustained pressure over Johnson's leadership of the anti-corruption unit.
"I think it's a direct fallout from that scrutiny. I think Ms Johnson has been feeling a huge amount of pressure in the last couple of months, more particularly since her appearance at the Ad Hoc Committee and the scrutiny that has now been directed to Idac and the case selection," Breytenbach said.
She added that Johnson's recent hospitalisation on the day she was due to testify illustrated the extent of the pressure she had been facing.
"I think the fact that she was rushed to hospital the day that she was required to testify sort of demonstrates that the pressure is just too much," she said.
Meanwhile, the Council for the Advancement of the South African Constitution (Casac) has called for an independent advisory panel to oversee the appointment of the next Idac head.
The organisation argues that the selection process should not rest solely with President Ramaphosa and has urged a transparent and independent appointment process.
Casac executive secretary Lawson Naidoo said Johnson's resignation was not unexpected given the allegations that have emerged against her.
"Her position quite clearly had become totally untenable. There's no way that she could have continued as the head of the leading anti-corruption agency in the country, given all of the allegations that have been levelled against her, and she has to face the consequences for all that conduct, and due process must be followed," Naidoo said.
He cautioned against drawing conclusions before the relevant processes have been completed.
"Whilst I think it would be premature to pre-judge those issues, we've got to wait for due process, and she must face the consequences of her actions in that regard," he added.
Johnson's resignation comes as the Madlanga Commission continues its investigation into allegations relating to corruption and governance, with further testimony expected in the coming days.