Ian Neilson has been appointed acting executive mayor of Cape Town, hours after the Democratic Alliance (DA) dramatically announced that veteran politician Patricia de Lille has ceased to be a member of the party, thereby ending her seven-year rule as mayor of the metropole.

A statement from the City said: "The Democratic Alliance has ceased the party membership of Patricia de Lille. This means she has lost her seat as a councillor and is therefore no longer the Executive Mayor of Cape Town, with immediate effect.

"In terms of the Municipal Structures Act, when the post of Executive Mayor is vacant, the Executive Deputy Mayor, who is elected by Council, automatically holds all Mayoral authority until such time as a new Executive Mayor is elected by Council."  

Neilson added: "As Acting Executive Mayor, I hold all the authority of the Mayoral seat and am committed to continuing to serve the residents of Cape Town with uninterrupted service delivery. Having served as the Executive Deputy Mayor for the past nine years, having been a City councillor for 22 years, and having been involved in the leadership of numerous administrations, I am fully apprised of the needs of the City and those we serve."

Natasha Mazzone, the deputy chairperson of the DA's federal council, early on Tuesday told a media briefing the "cessation of her membership" was based on the fact that De Lille breached the party's constitution by declaring her readiness to resign on a radio talk show late last month.

De Lille's assertion that she was "ready to walk away" sealed her fate, Mazzone said.

De Lille though, in trademark style, hit back and accused the party she has served for a decade and a half of having a regressive agenda.

Meanwhile, De Lille's mayoral committee has been dissolved with immediate effect due to her departure. "As it stands, at this moment, only the Speaker and I are political authorities in the City of Cape Town," Neilson said.

"I will now apply my mind in the coming days to appointing an interim Mayoral Committee that will serve until Council elects a new Executive Mayor."

The statement added that the City Manager has informed the Independent Electoral Commission of South Africa (IEC) about the Council vacancy. "We await due process to unfold and we will communicate further in due course.

"In the meantime, we assure the residents of Cape Town that the administration of the City is secure and we remain committed to the work we do to deliver services across the City."