Mbhazima Shilowa said he would help organise a convention for ANC rebels
The ex-premier of South Africa's Gauteng province has resigned from the ruling African National Congress (ANC) to join those calling for a new party.
Mbhazima Shilowa said the rebels, led by ex-Defence Minister Mosiuoa Lekota, would hold a convention on 2 November.
Mr Shilowa resigned as premier in protest at the ousting last month of Thabo Mbeki as president.
The ANC has been riven by splits between Mr Mbeki's supporters and those of ANC leader Jacob Zuma.
Mr Zuma won a bitter contest to replace Mr Mbeki in December.
Kgalema Motlanthe has been sworn in as the new president, but Mr Zuma is favourite to take over after elections next year.
Observers have said a split in the ANC has appeared increasingly likely, though a new party is unlikely to become a major power in South Africa unless it attracts political heavyweights.
The ANC suspended Mr Lekota this week, saying it would take similar action against anyone else from the party who threatened to establish an opposition movement.
Mr Shilowa broke the news of his defection at a press conference in Pretoria.
"I have decided to resign my membership from the ANC with immediate effect and to lend my support to the initiative by making myself available on a full-time basis as a convener and volunteer-in-chief together with comrade Mosiuoa," he said.
"I have taken this decision knowing fully well that I will be vilified," he said.
The new party would discuss constitutional reform and in particular whether, in the light of what happened to Mr Mbeki, South Africa's president should now be directly elected.
Mr Mbeki stood down after a judge suggested he had interfered in the prosecution of Mr Zuma on corruption charges, something the former South African leader denies.
Mr Lekota had accused the ANC's new leadership of arrogance, saying a split within the movement was "inevitable".
On Tuesday, Mr Zuma described party dissidents as charlatans, and said the ANC would act "very decisively" to rid the movement of what he described as factionalism.
"History has been extremely unkind to those who break away from the ANC," he said.