The merging of the Democratic Alliance (DA) and Agang SA will be aided by a joint technical committee, DA leader Helen Zille said on Tuesday.
"The truth is we have been interacting continuously … you look at different formats, you go through different options … We have an agreement politically, we just need the technical details worked out," Zille said.
On Tuesday, the DA held a much-anticipated press briefing, during which Zille announced that Agang SA leader Mamphela Ramphele would be the opposition party's presidential candidate.
"This is a game-changing moment for South Africa," she told reporters in Cape Town.
Zille said when she was elected DA leader in 2007, she set out a case for the realignment of politics in the country.
"I said then that political change will come about as the old political formations become obsolete. We can see this happening now."
Zille said the DA's leadership was drawn from across the political spectrum but they shared the same values: "The belief in an open-opportunity society for all characterised by non-racialism, a market economy, human rights and prosperity for every South African."
Talks between Agang SA and the DA had been happening "continuously", said Zille.
She said former president Nelson Mandela's death had changed many things for South Africa.
Ramphele added: "I am the bridge between my generation who fought for freedom and the new generation which now needs to lead.
"The ANC is fracturing along all the broad-church lines."
Even ANC supporters wanted political change, Ramphele said.
"Good people in the ANC have reached out to us, but are scared," she told reporters. "Good business leaders have reached out to us but are nervous."
Ramphele said millions of people wanted to make a different choice in this year's election.
"This is your government-in-waiting.
"I will not lead you, not lead us, nor lead South Africa on the path of conflict but of commitment – to you, to your children, to health, to high quality education and job opportunities," she said, flanked by Zille. – Sapa